Sea; it is a representative of a large series of species in which the
peculiarities of Termite life are exhibited in a comparatively simple
manner. There is no special caste of workers, consequently such work as is
done is carried on by the other members of the community, viz. soldiers,
and the young and adolescent. The habits of this species have recently been
studied in detail in Sicily by Grassi and Sandias.[289] The Insects dwell
in the branches and stems of decaying or even dead trees, where they
nourish themselves on those parts of the wood in which the process of decay
is not far advanced; they live in the interior of the stems, so that
frequently no sign of them can be seen outside, even though they may be
heard at work by applying the ear to a branch. They form no special
habitation, the interior of the branch being sufficient protection, but
they excavate or increase the natural cavities to suit their purposes. It
is said that they line the galleries with proctodaeal cement; this is
doubtful, but they form barricades and partitions where necessary, by
cementing together the proctodaeal products with matter from the salivary
glands or regurgitated from the anterior parts of the alimentary canal. The
numbers of a community only increase slowly and remain always small; rarely
do they reach 1000, and usually remain very much below this. The king and
queen move about, and their family increases but slowly. After fifteen
months of their union they may be surrounded by fifteen or twenty young; in
another twelve months the number may have increased to fifty, and by the
time it has reached some five hundred or upwards the increase ceases. This
is due to the fact that the fertility of the queen is at first progressive,
but ceases to be so. A queen three or four years old produces at the time
of maximum production four to six eggs a day. When the community is
small—during its first two years—the winged individuals that depart from it
are about eight or ten annually, but the numbers of the swarm augment with
the increase of the {364}population. The growth of the individuals is slow;
it appears that more than a year elapses between the hatching of the egg
and the development of the winged Insect. The soldier may complete its
development in less than a year; the duration of its life is not known;
that of the kings and queens must be four or five years, probably more.
After the winged Insects leave the colony they associate themselves in
pairs, each of which should, if all goes well, start a new colony.
The economy of _Termes lucifugus_, the only European Termite besides
_Calotermes flavicollis_, has been studied by several observers, the most
important of whom are Lespès[290] and Grassi and Sandias. This species is
much more advanced in social life than _Calotermes_ is, and possesses both
workers and soldiers (Fig. 231, 2, 3); the individuals are much smaller
than those of _Calotermes_. Burrows are made in wood of various kinds,
furniture being sometimes attacked. Besides making excavations this species
builds galleries, so that it can move from one object to another without
being exposed; it being a rule—subject to certain exceptions—that Termites
will not expose themselves in the outer air. This is probably due not only
to the necessity for protection against enemies, but also to the fact that
they cannot bear a dry atmosphere; if exposed to a drying air they speedily
succumb. Occasionally specimens may be seen at large; Grassi considers
these to be merely explorers. Owing to the extent of the colonies it is
difficult to estimate with accuracy the number of individuals composing a
community, but it is doubtless a great many thousands. Grassi finds the
economy of this species in Sicily to be different from anything that has
been recorded as occurring in other species; there is never a true royal
pair. He says that during a period of six years he has examined thousands
of nests without ever finding such a pair. In place thereof there are a
considerable number of complementary queens—that is, females that have not
gone through the full development to perfect Insects, but have been
arrested in various stages of development. In Fig. 231, Nos. 4 and 5 show
two of these abnormal royalties; No. 4 is comparatively juvenile in form,
while No. 5 is an individual that has been substituted in an orphaned nest,
and is nearer to the natural condition of perfect development. We have no
information as to whether any {365}development goes on in these individuals
after the state of royalty is assumed, or whether the differences between
these neoteinic queens are due to the state of development they may happen
to be in when adopted as royalties. Kings are not usually present in these
Sicilian nests; twice only has Grassi found a king, but he thinks that had
he been able to search in the months of August and September he would then
have found kings. It would appear therefore that the complementary kings
die, or are killed after they have fertilised the females. Parthenogenesis
is not thought to occur, as Grassi has found the spermathecae of the
complementary queens to contain spermatozoa.
[Illustration: FIG. 231.—Some of the forms of _Termes lucifugus_. 1, Young
larva; 2, adult worker; 3, soldier; 4, young complementary queen; 5, older
substitution queen; 6, perfect winged Insect. (After Grassi.)]
{366}The period of development apparently occupies from eighteen to
twenty-three months. At intervals swarms of a great number of winged
individuals leave the nest, and are usually promptly eaten up by various
animals. After swarming, the wings are thrown off, and sometimes two
specimens or three may be seen running off together; this has been supposed
to be preliminary to pairing, but Grassi says this is not the case, but
that the object is to obtain their favourite food, as we shall mention
subsequently.
Although these are the usual habits of _Termes lucifugus_ at present in
Sicily, it must not be concluded that they are invariable; we have in fact
evidence to the contrary. Grassi has himself been able to procure in
confinement a colony—or rather the commencement of one—accompanied by a
true royal pair; while Perris has recorded[291] that in the Landes he
frequently found a royal pair of _T. lucifugus_ under chips; they were
accompanied in nearly every case by a few eggs. And Professor Perez has
recently placed a winged pair of this species in a box with some wood, with
the result that after some months a young colony has been founded. It
appears probable therefore that this species at times establishes new
colonies by means of royal pairs derived from winged individuals, but after
their establishment maintains such colonies as long as possible by means of
complementary queens. It is far from improbable that distinctions as to the
use of true and complementary royalties may be to some extent due to
climatic conditions. In some localities _T. lucifugus_ has multiplied to
such an extent as to be very injurious, while in others where it is found
it has never been known to do so.
The Termitidae of Africa are the most remarkable that have yet been
discovered, and it is probably on that continent that the results of the
Termitid economy have reached their climax. Our knowledge of the Termites
of tropical Africa is chiefly due to Smeathman, who has described the
habits of several species, among them _T. bellicosus_. It is more than a
century since Smeathman travelled in Africa and read an account of the
Termites to the Royal Society.[292] His information was the first of any
importance about Termitidae that was given to the world; it is, as may be
well understood, deficient in many details, but is nevertheless of great
value. Though his statements have been doubted they are truthful, and have
been confirmed by Savage.[293]
{367}[Illustration: FIG. 232.—Royal cell of _Termes bellicosus_, partially
broken open to show the queen and her attendants. (After Smeathman.) B,
Antenna of the queen; _b_, _b_, line of entrances to the cell; A, A, an
entrance, in this line, closed by the Termites. × ⅞.]
{368}_T. bellicosus_ forms buildings comparable to human dwellings; some of
them being twenty feet in height and of great solidity. In some parts of
West Africa these nests were, in Smeathman's time, so numerous that they
had the appearance of villages. Each nest was the centre of a community of
countless numbers of individuals; subterranean passages extended from them
in various directions. The variety of forms in one of these communities has
not been well ascertained, but it would seem that the division of labour is
carried to a great extent. The soldiers are fifteen times the size of the
workers. The community is dependent on one royal couple. It is the opinion
of the natives that if that couple perish so also does the community; and
if this be correct we may conclude that this species has not a perfect
system of replacing royal couples. The queen attains an almost incredible
size and fertility. Smeathman noticed the great and gradual growth of the
abdomen, and says it enlarges "to such an enormous size that an old queen
will have it increased so as to be fifteen hundred or two thousand times
the bulk of the rest of her body, and twenty or thirty thousand times the
bulk of a labourer, as I have found by carefully weighing and computing the
different states." He also describes the rate at which the eggs are
produced, saying that there is a constant peristaltic movement[294] of the
abdomen, "so that one part or other alternately is rising and sinking in
perpetual succession, and the matrix seems never at rest, but is always
protruding eggs to the amount (as I have frequently counted in old queens)
of sixty in a minute, or eighty thousand and upward in one day of
twenty-four hours."
This observer, after giving an account of the great swarms of perfect
winged Insects that are produced by this species, and after describing the
avidity with which they are devoured by the Hymenopterous ants and other
creatures, adds: "I have discoursed with several gentlemen upon the taste
of the white ants; and on comparing notes we have always agreed that they
are most {369}delicious and delicate eating. One gentleman compared them to
sugared marrow, another to sugared cream and a paste of sweet almonds."
From the preceding brief sketch of some Termitidae we may gather the chief
points of importance in which they differ from other Insects, viz. (1) the
existence in the community of individuals—workers and soldiers—which do not
resemble their parents; (2) the limitation of the reproductive power to a
single pair, or to a small number of individuals in each community, and the
prolongation of the terminal period of life. There are other social Insects
besides Termitidae: indeed, the majority of social Insects—ants, bees, and
wasps—belong to the Order Hymenoptera, and it is interesting to note that
analogous phenomena occur in them, but nevertheless with such great
differences that the social life of Termites must be considered as totally
distinct from that of the true ants and other social Hymenoptera.
DEVELOPMENT.—Social Insects are very different to others not only in the
fact of their living in society, but in respect of peculiarities in the
mode of reproduction, and in the variety of habits displayed by members of
a community. The greatest confusion has arisen in reference to Termitidae
in consequence of the phenomena of their lives having been assumed to be
similar to those of Hymenoptera; but the two cases are very different,
Hymenoptera passing the early parts of their lives as helpless maggots, and
then undergoing a sudden metamorphosis to a totally changed condition of
structure, intelligence, and instinct.
The development of what we may look on as the normal form of
Termitidae—that is, the winged Insects male and female—is on the whole
similar to that we have sketched in Orthoptera; the development in earwigs
being perhaps the most similar. The individuals of Termitidae are, however,
in the majority of cases if not in all, born without eyes; the
wing-rudiments develop from the thoracic terga as shorter or longer lobes
according to the degree of maturity; as in the earwigs the number of joints
in the antennae increases as development advances. All the young are, when
hatched, alike, the differences of caste appearing in the course of the
subsequent development; the most important of these differences are those
that result in the production of two special classes—only met with in
social Insects—viz. worker and soldier. Of these the workers are
individuals whose {370}development is arrested, the sexual organs not going
on to their full development, while other organs, such as the eyes, also
remain undeveloped; the alimentary canal and its adjuncts occupy nearly the
whole of the abdominal cavity. The adult worker greatly resembles—except in
size—the young. Grassi considers that the worker is not a case of simple
arrest of development, but that some deviation accompanies the arrest.
The soldier also suffers an arrest of development in certain respects
similar to the worker; but the soldier differs in the important fact that
the arrest of the development of certain parts is correlative with an
extraordinary development of the head, which ultimately differs greatly
from those of either the worker or of the sexual males and females.
[Illustration: FIG. 233.—The pairs of mandibles of different adult
individuals of _Termes_ sp. from Singapore. A, Of worker; B, of soldier; C,
of winged male; D, of winged female.]
SOLDIER.—All the parts of the head of the soldier undergo a greater or less
change of form; even the pieces at its base, which connect it by means of
the cervical sclerites with the prothorax, are altered. The parts that
undergo the greatest modification are the mandibles (Fig. 233, B); these
become much enlarged in size and so much changed in form that in a great
many species no resemblance to the original shape of these organs can be
traced. It is a curious fact that the specific characters are better
expressed in these superinduced modifications than they are in any other
part of the organisation (except, perhaps, the wings). The soldiers are not
alike in any two species of Termitidae so far as we know, and it seems
impossible to ascribe the differences that exist between the soldiers of
different species of Termitidae to special adaption for the work they have
to perform. Such a suggestion is justifiable only in the case of the Nasuti
(Fig. 234, 1), where the front of the head is prolonged into a point: a
duct opens at the extremity of this point, from which is exuded a fluid
that serves as a cement for {371}constructing the nest, and is perhaps also
used to disable enemies. Hence the prolongation and form of the head of
these Nasuti may be fairly described as adaptation to useful ends. As
regards the great variety exhibited by other soldiers—and their variety is
much greater than it is in the Nasuti—it seems at present impossible to
treat it as being cases of special adaptations for useful purposes. On the
whole it would be more correct to say that the soldiers are very dissimilar
in spite of their having to perform similar work, than to state that they
are dissimilar in conformity with the different tasks they carry on.
[Illustration: FIG. 234.—Soldiers of different species of Termites. (After
Hagen.) 1, _Termes armiger_; 2, _T. dirus_; 3, _Calotermes flavicollis_; 4,
_T. bellicosus_; 5, _T. occidentis_; 6, _T. cingulatus (?)_; 7, _Hodotermes
quadricollis (?)_; 8, _T. debilis (?)_, Brazil.]
The Termite soldier is a phenomenon to which it is difficult to find a
parallel among Insects. The soldier in the true ants is usually not
definitely distinguished from the worker, but it is possible that in the
leaf-cutting ants, the so-called soldier may prove to be more similar in
its nature to the Termite soldier. The soldiers of any one species of
Termite are apparently {372}extremely similar to one another, and there are
no intermediates between them and the other forms, except in the stages of
differentiation. But we must recollect that but little is yet known of the
full history of any Termite community, and it is possible that soldiers
which in the stage of differentiation promise to be unsatisfactory may be
killed and eaten,—indeed there is some evidence to this effect. There is
too in certain cases some difference—larger or smaller size being the most
important—between the soldiers of one species, which may possibly be due to
the different stage of development at which their differentiation
commenced.
It would at present appear that, notwithstanding the remarkable difference
in structure of the soldiers and workers of the white ants, there is not a
corresponding difference of instinct. It is true that soldiers do more of
certain things than workers do, and less of others, but this appears to be
due solely to their possession of such very different structures; and we
are repeatedly informed by Grassi that all the individuals in a community
take part, so far as they are able, in any work that is going on, and we
find also in the works of other writers accounts of soldiers performing
acts that one would not have expected from them. The soldiers are not such
effective combatants as the workers are. Dudley and Beaumont indeed
describe the soldiers as merely looking on while the workers fight.[295] So
that we are entitled to conclude that the actions of the soldiers, in so
far as they differ from those of the rest of the community, do so because
of the different organisation and structures of these individuals. We
shall, when speaking of food, point out that the condition of the soldier
in relation to food and hunger is probably different from that of the other
forms.
VARIOUS FORMS OF A COMMUNITY.—The soldiers and workers are not the only
anomalous forms found in Termitid communities; indeed on examining a large
nest a variety of forms may be found that is almost bewildering. Tables
have been drawn up by Grassi and others showing that as many as fifteen
kinds may be found, and most of them may under certain circumstances
coexist. Such tables do not represent the results of actual examination in
any one case, and they by no means adequately represent the number that,
according to the most recent observations of Grassi, may be present; but we
give one taken {373}from Grassi, as it conveys some idea of the numerous
forms that exist in certain communities. In this table the arrangement,
according to A, B, C, D, E, represents successive stages of the
development:—
_Forms of Termes lucifugus_. (After Grassi.) _Zool. Anz._ xii. 1889, p.
360.
A 1. Young, undifferentiated larvae.
|
+----------------------+--------------------------+
| | |
B 2. Larvae that will 3. Larvae that will 4. Reserves for royal
not mature the sexual mature the sexual specimens:(only present
organs. organs. when 14, 15, and 11 are
| | wanting, or when 14 and
| | 15 are present in
| | insufficient numbers).
| |
+-----+-----+ +------------+-------------=
| | | | |
C 5. Larvae 6. Larvae 9. Nymphs of 10. Nymphs 11. Reserves for
of soldiers. of workers. the first of the royal pairs (only
| | form. second present when 14,
| | | form. 15, and 4 are
| | | | wanting, or when
| | | | the two latter
D 7. Soldiers. 8. Workers. +----+-----+ | are present in
| | | insufficient
12. Winged 13. Reserve | numbers).
Insects royal pairs? |
| |
E 14. True royal 15. Substitution
couple. royal pairs.
On inspecting this table it will be perceived that the variety of forms is
due to three circumstances—(1) the existence of castes that are not present
in ordinary Insects; (2) the coexistence of young, of adolescents, and of
adults; and (3) the habit the Termites have of tampering with forms in
their intermediate stages, the result of which may be the substitution of
neoteinic individuals in place of winged forms.
This latter procedure is far from being completely understood, but to it
are probably due the various abnormal forms, such as soldiers with
rudiments of wings, that have from time to time been discovered in Termite
communities, and have given rise to much perplexity.
In connexion with this subject we may call attention to the necessity, when
examining Termite nests, of taking cognisance of the fact that more than
one species may be present. Bates found different Termites living together
in the Amazons Valley, and Mr. Haviland has found as many as five species
of Termitidae and three of true ants in a single mound in South Africa. In
this latter case observation showed that, though in such close proximity,
there was but little further intimacy between the species. There are,
however, true inquiline, or guest, Termites, {374}of the genus _Eutermes_,
found in various parts of the world living in the nests of other
Termitidae.
ORIGIN OF THE FORMS.—The interest attaching to the various forms that exist
in Termites, more particularly to the worker and soldier, is evident when
we recollect that these never, so far as we know, produce young. In the
social Hymenoptera it has been ascertained that the so-called neuters
(which in these Insects are always females) can, and occasionally do,
produce young, but in the case of the Termites it has never been suggested
that the sexual organs of the workers and soldiers, whether male or female,
ever become fruitful; moreover, the phenomena of the production of young by
the white ants are of such a nature as to render it in the highest degree
improbable that either workers or soldiers ever take any direct part in it.
Now the soldier is extremely different from the sexual individuals that
produce the young, and seeing that its peculiarities are not, in the
ordinary sense of the word, hereditary, it must be of great interest to
ascertain how they arise.
Before stating the little information we possess on this subject, it is
necessary to reiterate what we have already said to the effect that the
soldiers and workers are not special to either sex, and that all the young
are born alike. It would be very natural to interpret the phenomena by
supposing the workers to be females arrested in their development—as is the
case in social Hymenoptera—and by supposing the soldiers to be males with
arrested and diverted development.
The observations already made show that this is not the case. It has been
thoroughly well ascertained by Lespès and Fritz Müller that in various
species of _Calotermes_ the soldiers are both males and females. Lespès and
Grassi have shown that the workers of _Termes lucifugus_ are of male and
female sex, and that this is also true of the soldiers. Although the view
of the duality of the sexes of these forms was received at first with
incredulity, it appears to be beyond doubt correct. Grassi adds that in all
the individuals of the workers and soldiers of _Termes lucifugus_ the
sexual organs, either male or female, are present, and that they are in the
same stage of development whatever the age of the individual. This
statement of Grassi's is of importance because it seems to render
improbable the view that the difference of form of the soldier and worker
arises from the arrest of the {375}development of their sexual organs at
different periods. The fact that sex has nothing whatever to do with the
determination of the form of workers and soldiers may be considered to be
well established.
The statement that the young are all born alike is much more difficult to
substantiate. Bates said that the various forms could be detected in the
new-born. His statement was made, however, merely from inspection of the
nests of species about which nothing was previously known, and as it is
then very difficult to decide that a specimen is newly hatched, it is
probable that all he meant was that the distinction of workers, soldiers,
and sexual forms existed in very small individuals—a statement that is no
doubt correct. Other observers agree that the young are in appearance all
alike when hatched, and Grassi reiterates his statement to this effect.
Hence it would appear that the difference of form we are discussing arises
from some treatment subsequent to hatching. It may be suggested,
notwithstanding the fact that the young are apparently alike when hatched,
that they are not really so, but that there are recondite differences which
are in the course of development rendered conspicuous. This conclusion
cannot at present be said with certainty to be out of the question, but it
is rendered highly improbable by the fact ascertained by Grassi that a
specimen that is already far advanced on the road to being an ordinary
winged individual can be diverted from its evident destination and made
into a soldier, the wings that were partially developed in such a case
being afterwards more or less completely absorbed. This, as well as other
facts observed by Grassi, render it probable that the young are truly, as
well as apparently, born in a state undifferentiated except as regards sex.
Fig. 230 (p. 363) is designed to illustrate Grassi's view as to this
modification; the individual A is already far advanced in the direction of
the winged form C, but can nevertheless be diverted by the Termites to form
the adult soldier B.
According to the facts we have stated, neither heredity nor sex nor arrest
of development are the causes of the distinctions between worker and
soldier, though some arrest of development is common to both; we are
therefore obliged to attribute the distinction between them to other
influences. Grassi has no hesitation in attributing the anatomical
distinctions that arise between the soldiers, workers, and winged forms to
alimentation. {376}Food, or the mode of feeding, or both combined, are,
according to the Italian naturalist, the source of all the distinctions,
except those of sex, that we see in the forms of any one species of
Termite.
FEEDING.—Such knowledge as we possess of the food-habits of Termitidae is
chiefly due to Grassi; it is of the very greatest importance, as giving a
clue to much that was previously obscure in the Natural History of these
extraordinary creatures.
In the abodes of the Termites, notwithstanding the enormous numbers of
individuals, cleanliness prevails; the mode by which it is attained appears
to be that of eating all refuse matter. Hence the alimentary canal in
Termitidae contains material of various conditions of nutritiveness. These
Insects eat their cast skins and the dead bodies of individuals of the
community; even the material that has passed through the alimentary canal
is eaten again, until, as we may presume, it has no further nutritive
power. The matter is then used for the construction of their habitations or
galleries, or is carried to some unfrequented part of the nest, or is
voided by the workers outside of the nests; the pellets of frass, _i.e._
alimentary rejectamenta, formed by the workers frequently betraying their
presence in buildings when none of the Insects themselves are to be seen.
The aliments of _Calotermes flavicollis_ are stated by Grassi and Sandias
to be as follows: (1) wood; (2) material passed from the posterior part of
the alimentary canal or regurgitated from the anterior part; (3) the matter
shed during the moults; (4) the bodies of other individuals; (5) the
secretion of their own salivary glands or that of their fellows; (6) water.
Of these the favourite food is the matter passed from the posterior part of
the alimentary canal. We will speak of this as proctodaeal food. When a
_Calotermes_ wishes food it strokes the posterior part of another
individual with the antennae and palpi, and the creature thus solicited
yields, if it can, some proctodaeal food, which is then devoured. Yielding
the proctodaeal food is apparently a reflex action, as it can be induced by
friction and slight pressure of the abdomen with a small brush. The
material yielded by the anterior part of the alimentary canal may be called
stomodaeal product. It makes its appearance in the mouth in the form of a
microscopic globule that goes on increasing in size till about one
millimetre in diameter, when it is {377}either used for building or as food
for another individual. The mode of eating the ecdysial products has also
been described by Grassi and Sandias. When an individual is sick or
disabled it is frequently eaten alive. It would appear that the soldiers
are great agents in this latter event, and it should be noticed that owing
to their great heads and mandibles they can obtain food by other means only
with difficulty. Since they are scarcely able to gnaw wood, or to obtain
the proctodaeal and stomodaeal foods, their condition may be considered to
be that of permanent hunger, only to be allayed by carnivorous proceedings.
When thrown into a condition of excitement the soldiers sometimes exhibit a
sort of Calotermiticidal mania, destroying with a few strokes five or six
of their fellows. It is, however, only proper to say that these strokes are
made at random, the creature having no eyes. The carnivorous propensities
of _Calotermes_ are apparently limited to cannibalism, as they slaughter
other white ants (_Termes lucifugus_) but never eat them.
The salivary food is white and of alkaline nature; when excreted it makes
its appearance on the upper lip. It is used either by other individuals or
by the specimen that produced it; in the latter case it is transferred to
the lower lip and swallowed by several visible efforts of deglutition. The
aliments we have mentioned are made use of to a greater or less extent by
all the individuals except the very young; these are nourished only by
saliva: they commence taking proctodaeal and stomodaeal food before they
can eat triturated wood.
ROYAL PAIRS.—The restriction of the reproductive powers of a community to a
single pair (or to a very restricted number of individuals) occurs in all
the forms of social Insects, and in all of them it is concomitant with a
prolongation of the reproductive period far exceeding what is natural in
Insects. We are not in a position at present to say to what extent the
lives of the fertile females of Termitidae are prolonged, there being great
difficulties in the way of observing these Insects for long periods owing
to their mode of life; living, as they do, concealed from view, light and
disturbance appear to be prejudicial to them. We have every reason to
believe, however, that the prolongation extends as a rule over several
years, and that it is much greater than that of the other individuals of
the community, although the lives of even these latter are longer than is
usual in Insects; but this {378}point is not yet satisfactorily
ascertained. As regards the males there is reason to think that
considerable variety as to longevity prevails. But the belief is that the
royal males of Termitidae also form an exception to other Insects in the
prolongation of the terminal periods of their lives. In Hymenoptera, male
individuals are profusely produced, but their lives are short, and their
sole duty is the continuation of the species by a single act. We have seen
that Grassi is of opinion that a similar condition of affairs exists at
present with _Termes lucifugus_ in Sicily, but with this exception it has
always been considered that the life of the king Termite is, roughly
speaking, contemporaneous with that of the queen; it is said that in
certain species the king increases in bulk, though not to an extent that
can be at all compared with the queen.
It must be admitted that the duration of life of the king has not been
sufficiently established, for the coexistence of a king with a queen in the
royal cell is not inconsistent with the life of the king being short, and
with his replacement by another. Much that is imaginary exists in the
literature respecting Termites, and it is possible that the life of the
king may prove to be not so prolonged as has been assumed.
[Illustration: FIG. 235.—Royal pair of _Termes_ sp. from Singapore, taken
out of royal cell. A, A, King, lateral and dorsal views; B, B, queen,
dorsal and lateral views. Natural sizes.]
Returning to the subject of the limitation of the reproduction of the
community to a single pair, we may remark that _a priori_ one would suppose
such a limitation to be excessively unfavourable to the continuation of the
species; and as it nevertheless is the fact that this feature is almost, if
not quite, without exception {379}in Insect societies, we may conclude that
it is for some reason absolutely essential to Insect social life. It is
true that there are in Termitidae certain partial exceptions, and these are
so interesting that we may briefly note them. When a royal cell is opened
it usually contains but a single female and male, and when a community in
which royal cells are not used is inspected it is usually found that here
also there are present only a single fertile female and a single king.
Occasionally, however, it happens that numerous females are present, and it
has been noticed that in such cases they are not fully matured females, but
are imperfect, the condition of the wings and the form of the anterior
parts of the body being that of adolescent, not adult Insects. It will be
recollected that the activity of a community of Termites centres round the
great fertility of the female; without her the whole community is, as
Grassi graphically puts it, orphaned; and the observations of the Italian
naturalist make it clear that these imperfect royalties are substitution
queens, derived from specimens that have not undergone the natural
development, but have been brought into use to meet the calamity of
orphanage of the community. The Termites apparently have the power of
either checking or stimulating the reproductive organs apart from other
organs of the body, and they appear to keep a certain number of individuals
in such a condition that in case of anything going wrong with the queen,
the reserves may be brought as soon as possible into a state of
reproductive activity. The individuals that are in such a condition that
they can become pseudo-royalties are called complementary or reserve
royalties, and when actually brought into use they become substitution
royalties. It is not at present quite clear why the substitution royalties
should be in such excess of numbers as we have stated they were in the case
we have figured (Fig. 236), but it may be due to the fact that when the
power of the community is at a certain capacity for supporting young a
single substitution royalty would not supply the requisite producing power,
and consequently the community adopts a greater number of the substitution
forms. Termites are utterly regardless of the individual lives of the
members of the community, and when the reproductive powers of the company
of substitution royalties become too great, then their number is reduced by
the effective method of killing and eating them.
According to Grassi's observations, the communities of _Termes
{380}lucifugus_ are now kept up in Sicily almost entirely by substitution
royalties; the inference being that the age of each community has gone
beyond the capacity for life of any single royal queen.
The substitution royalties are, as we have said, called neoteinic (νεος,
youthful, τείνω, to belong to), because, though they carry on the functions
of adult Insects, they retain the juvenile condition in certain respects,
and ultimately die without having completed the normal development. The
phenomenon is not quite peculiar to Insects, but occurs in some other
animals having a well-marked metamorphosis, notably in the Mexican
Axolotl.[296]
[Illustration: FIG. 236.—Pair of neoteinic royalties, taken from the royal
chamber of _Termes_ sp. at Singapore by Mr. G. D. Haviland. The queen was
one of thirteen, all in a nearly similar state. A, king; B, C, queen.]
A point of great importance in connexion with the neoteinic royalties is
that they are not obtained from the instar immediately preceding the adult
state, but are made from Insects in an earlier stage of development. The
condition immediately preceding the adult state is that of a nymph with
long wing-pads; such specimens are not made into neoteinic royalties, but
nymphs of an earlier stage, or even larvae, are preferred. It is apparently
by an interference with one of these earlier stages of development that the
"nymphs of the second form," which have for long been an enigma to
zoologists, are produced.
POST-METAMORPHIC GROWTH.—The increase of the fertility of the royal female
is accompanied by remarkable phenomena of growth. Post-metamorphic growth
is a phenomenon almost unknown in Insect life, except in these Termitidae;
distension not infrequently occurs to a certain extent in other Insects,
and {381}is usually due to the growth of eggs inside the body, or to the
repletion of other parts. But in Termitidae there exists post-metamorphic
growth of an extensive and complex nature; this growth does not affect the
sclerites (_i.e._ the hard chitinous parts of the exo-skeleton), which
remain of the size they were when the post-metamorphic growth commenced,
and are consequently mere islands in the distended abdomen (Fig. 236, B,
C). The growth is chiefly due to a great increase in number and size of the
egg-tubes, but there is believed to be a correlative increase of various
other parts of the abdominal as distinguished from the anterior regions of
the body. A sketch of the distinctions existing between a female of a
species at the time of completion of the metamorphosis and at the period of
maximum fertility does not appear to have been yet made.
NEW COMMUNITIES.—The progress of knowledge in respect of Termitidae is
bringing to light a quite unexpected diversity of habits and constitution.
Hence it is premature to generalise on important matters, but we may refer
to certain points that have been ascertained in connexion with the
formation of new communities. The duration of particular communities and
the modes in which new ones are founded are still very obscure. It was
formerly considered that swarming took place in order to increase the
number of communities, and likewise for promoting crossing between the
individuals of different communities. Grassi, however, finds as the result
of his prolonged observations on _Termes lucifugus_ that the swarms have no
further result than that the individuals composing them are eaten up. And
Fritz Müller states[297] that in the case of the great majority of forms
known to him the founding of a colony by means of a pair from a swarm would
be just about as practicable as to establish a new colony of human beings
by placing a couple of newly-born babes on an uninhabited island. It was
also thought that pairs, after swarming, re-entered the nests and became
royal couples. It does not, however, appear that any one is able to produce
evidence of such an occurrence. The account given by Smeathman of the
election of a royal couple of _Termes bellicosus_ is imperfect, as, indeed,
has already been pointed out by Hagen. It suggests, however, that a winged
pair after leaving the nest do again enter it to become king {382}and
queen. The huge edifices of this species described by Smeathman are clearly
the result of many years of labour, and at present substitution royalties
are not known to occur in them, so that it is not improbable Smeathman may
prove to be correct even on this point, and that in the case of some
species mature individuals may re-enter the nest after swarming and may
become royal couples. On the whole, however, it appears probable that
communities of long standing are kept up by the substitution royalty
system, and that new communities when established are usually founded by a
pair from a swarm, which at first are not in that completely helpless
condition to which they come when they afterwards reach the state of
so-called royalty. Grassi's observations as to the sources of food remove
in fact one of the difficulties that existed previously in regard to the
founding of new colonies, for we now know that a couple may possibly bear
with them a sufficient supply of proctodaeal and stomodaeal aliment to last
them till workers are hatched to feed them, and till soldiers are developed
and the community gradually assumes a complex condition. Professor Perez
has recently obtained[298] the early stages of a community from a winged
pair after they had been placed in captivity, unattended by workers.
Müller's observation, previously quoted, is no doubt correct in relation to
the complete helplessness of royal pairs after they have been such for some
time; but that helplessness is itself only gradually acquired by the royal
pair, who at first are able to shift for themselves, and produce a few
workers without any assistance.
ANOMALOUS FORMS.—Müller has described a _Calotermes_ under the name of _C.
rugosus_, which is interesting on account of the peculiar form of the young
larva, and of the changes by which it subsequently becomes similar in form
to other species of the genus. We represent the development of this larva
in Fig. 237. We may call attention to the fact that this figure illustrates
the large size of the paunch, which is so extraordinary in some of the
states of the Termitidae.
It will be recollected that the genus _Calotermes_ is destitute of workers.
There is another genus, _Anoplotermes_, in which the reverse condition
prevails, and the soldier is absent; this is the only case yet known in
which such a state of affairs exists. {383}The species is called _A.
pacificus_ by Fritz Müller; it differs from other Termitidae in possessing
a proventriculus destitute of triturating ridges. The nests of this species
are utilised by a little _Eutermes_ (_E. inquilinus_ Müller) for its own
advantage; whether by first destroying the _Anoplotermes_ or whether by
merely taking possession of the nests abandoned by their owners is not
known. It is a most remarkable fact that the _Eutermes_ resembles the
_Anoplotermes_ so extremely that the two can scarcely be distinguished,
though anatomically they are quite different. The resemblance is indeed so
great that it deceived Von Jhering into supposing that the two genera were
alternate generations of a single species, one generation possessing
soldiers, the other being without them. Subsequently, by anatomical
investigation, he recognised[299] the error into which he had fallen—an
error that, under such peculiar circumstances, was quite pardonable.
[Illustration: FIG. 237.—Changes in external form of the young larva of
_Calotermes rugosus_. A, Newly hatched with nine joints in antennae, × 8;
B, older larva with ten joints, × 8; C, next stage with eleven joints, × 8;
D, larva with twelve joints; the position of the parts of the alimentary
canal are shown—_v_, crop; _m_, stomach; _b_, paunch; _e_, intestine; _r_,
dorsal vessel, × 16/3 (After Fritz Müller.)]
Hagen has suggested[300] that _Hodotermes japonicus_ never produces winged
forms. Very little, however, is actually known as to this species.
MARCHING AND HARVESTING TERMITES.—Smeathman alluded to a remarkable
_Termes_ seen by him in Africa, giving it the name of _T. viarum_. Nothing
further is known of this Insect, which, according to Smeathman's account,
may possibly be the most remarkable of the family. _T. viarum_ is said to
be larger than _T. bellicosus_, and was discovered issuing in large numbers
from a hole in the ground and marching in columns consisting of workers
directed by soldiers of enormous size, some of whom {384}climbed up plants
and gave audible signals to the army, which immediately responded with a
hissing noise and by increasing their pace with the utmost hurry; they
continued marching by the spot where Smeathman observed them for upwards of
an hour. He was not able to find their nests, and no specimens have been
preserved; both soldiers and workers possessed eyes. Marching in this way
by daylight is contrary to the nature of ordinary Termites, and some doubt
has existed as to the correctness of Smeathman's observation, which has in
fact remained for upwards of a century without confirmation.
[Illustration: FIG. 238.—Eyed, grass-cutting Termite, _Hodotermes
havilandi_, A, soldier; B, worker. South Africa. In life the head is
carried horizontally, so the piece of grass sticks up like a flag-pole.]
Mr. G. D. Haviland has, however, this year discovered in Natal a Termite
which shows that there are species in Africa of the kind described by
Smeathman, the workers and soldiers being possessed of facetted eyes. Mr.
Haviland states that the workers of this species issue from holes in the
ground during the heat of the day and cut grass both dead and green. They
carry it, in lengths of about two inches, to the mouths of the holes, often
leaving it there and going at once to fetch more. Under acacia bushes they
carry acacia leaflets as well as grass. In the middle of the day more grass
accumulates at the entrance to the holes than can be taken in, but as the
heat of the day diminishes the workers cease to forage and take in the
accumulation. When the grass is all in they sometimes close the mouth of
the hole with moistened pellets of earth brought in their mouths. The
soldiers remain in the holes; when disturbed they jerk themselves like
soldiers of other species to frighten away the intruder; when they bite,
their grip is very tenacious. The holes are about ⅓ of an inch in diameter,
and there are usually several of them a few yards apart; around each
{385}of them is a patch over which the grass has been cut quite short. Mr.
Haviland followed these holes by digging for a distance of 20 feet and to a
depth of 5½ feet; they remain uniform in size except that near the entrance
there may be one or two chambers in which the grass is temporarily stored,
but these do not hold more than would be collected in an hour or two. As
the burrow descends it is occasionally joined by another, and at the point
of junction there is usually a considerable widening. Sometimes they run
straight for 6 or 7 feet, sometimes they curve abruptly, sometimes they are
nearly horizontal, but near the mouth may be almost vertical in direction.
These Termites are very local, but the specimens are numerous when found.
Mr. Haviland dug for these Insects at two places on the Tugela river, one
of them being at Colenso. It is much to be regretted that he was unable to
reach the nest. We figure a soldier selected from specimens sent by Mr.
Haviland to the Cambridge University Museum. This Insect is apparently much
smaller than Smeathman's _T. viarum_. Other species of Termitidae have been
described[301] as forming underground tunnels in Africa, but none of the
species have yet been satisfactorily identified.
It was stated by Smeathman that some species of Termites had chambers in
their habitations in which grew a kind of fungus used by the Insects for
food; Mr. Haviland is able to confirm Smeathman in this particular; he
having found fungus-chambers in the nests of more than one species both in
Singapore and South Africa (Fig. 240).
HABITATIONS.—In nothing do Termites differ more than in the habitations
they form. Sometimes, as we have mentioned in the case of _Calotermes_,
there is no real structure formed; only a few barriers being erected in
burrows or natural hollows in wood. In other cases very extensive
structures are formed, so that the work of the Termites becomes a
conspicuous feature in the landscape. This is of course only the case in
regions that are not much interfered with by man; the great dwellings
spoken of by Smeathman and others soon disappear from the neighbourhood of
settlements, but in parts of Africa and in Australia large dwellings are
still formed by these creatures. In the latter part of the world there
exists a very remarkable one, formed by an {386}undetermined species called
by the officers and crew of her Majesty's ship _Penguin_ the "compass ant."
The outline of one of the structures formed by this Termite we represent in
Fig. 239. Mr. J. J. Walker, to whom we are indebted for the sketch from
which this figure is taken, has also favoured us with the following extract
from his diary, of date 4th August 1890: "The most interesting feature in
the scenery (about forty miles inland from Port Darwin) was the constant
succession of huge mounds raised by the Termites, of which I had seen some
comparatively small examples in my rambles near Port Darwin; but these
exceeded in dimensions all I had ever seen. The most frequent as well as
the largest kind was usually of a reddish or ferruginous colour outside,
and generally almost cylindrical in shape with obtusely-pointed top, but
nearly always more or less weather-worn, with great irregular buttresses
and deep ruts down the sides; many of them look like ruined towers in
miniature. Their usual height was from 8 to 10 feet, but many were much
higher, and some attained an (estimated) elevation of at least 20 feet.
Another kind, seen only in one or two places along the line, was of a much
more singular character; they averaged only 4 to 5 feet high, were built of
a dark-gray mud, and in shape were like thin flat wedges set upright (see
Fig. 239), reminding one of tombstones in a churchyard. But the most
remarkable feature about these mounds was that they had all the same
orientation, viz. with the long faces of the wedge pointing nearly north
and south. Why this is so I am quite at a loss to imagine, and I much
regret that I had no opportunity of closely examining these most singular
structures. A third kind of mound, usually not exceeding 2 feet in height,
was of a simple, acute, conical figure, and generally of a gray colour
somewhat paler than the last."
[Illustration: FIG. 239.—Termitarium of compass or meridian Termite of
North Australia. A, face extending south and north; B, cross-section.]
The material used for the construction of the dwellings is either earth,
wood, or the excrement of the Termites. The huge edifices mentioned by
Smeathman are composed of earth cemented {387}together so as to look like
stone or brick, and the buildings appear to be almost as strong as if they
were actually constructed with these materials. In many cases the substance
used is comminuted wood that has passed one or more times through the
alimentary canal of the Insects, and may therefore be called excrement.
Whether the stone-like material is made from earth that has passed through
the alimentary canal or from grains gathered for the purpose has not been
well ascertained. In any case the material is cemented together by means of
the secretions of glands. Dudley and Beaumont have described the process of
construction, in a species observed by them, saying that earth is brought
and placed in position by the mandibles, and cemented by liquid from the
abdomen.[302] Von Jhering says[303] that some species form the exterior
walls of their dwellings of stone-like material, but make use of woody
matter for the construction of the interior. Smeathman has described the
nest of _Termes bellicosus_. The whole of the very strong external wall
consists of clay-like material, cemented by the secretions of the Termites
to a very firm consistence. The royal cell is built of the same material as
the framework of the nest; whilst the nurseries in which the young are
chiefly found are built of woody material, and are always covered with a
kind of mould—the mycelium of a fungus—and plentifully sprinkled with small
white bodies, which, under the microscope, are found to be filled with a
number of oblong, spore-like cells.
[Illustration: FIG. 240.—Fragment of Termitarium of _Termes angustatus_, S.
Africa, showing fungus chambers and orifices of communication.]
These nurseries rest on the clay-like framework of the nest, but are not
attached thereto; they in no way support it, or one another, indeed they
have the appearance of being constantly added to on their upper margins and
constantly eaten away on their under parts. Fig. 240 represents the
appearance of the upper boundary of a nursery taken from a nest of _Termes
angustatus_. The small white bodies, mentioned above, have disappeared: the
mycelium of the fungus, though not shown in the {388}figure, is still
visible on the specimen from which it was drawn, and gives rise to a
whitish, glaucous appearance.
In various parts of the world nests formed on trees by Termites are to be
seen; these tree nests are, it would appear, in some cases only parts of a
community, and are connected with the main body by galleries. In other
cases nests are formed in various positions of advantage; Messrs. Hubbard
and Hagen have given us an account[304] of some of these—probably the work
of _Eutermes ripperti_—as seen in Jamaica. They describe the nests as
spherical or conical masses, looking externally as if composed of loamy
earth; they are placed on trees, fences, or walls; they vary in size from
that of a man's fist to that of a hogshead; they appear to be composed of
finely comminuted wood fastened together by saliva. These nests are formed
on the same principle as those of the wasps that make nests hanging to
trees and bushes, as they consist of an external protecting envelope
covering a comb-like mass in the interior. At the bottom of the nest there
is a covered gallery leading to the earth, where the main nest appears to
be situate; galleries also are constructed so as to lead to the tops of
trees and other places, in such a manner that the Termite can still keep up
its peculiarity of working and travelling in tunnels and yet roam over a
large area; the activity of these Termites continues day and night. In each
nest there is a queen, who lays eggs that are removed by the worker
Termites to the bottom of the nest. The young are fed on a prepared food,
consisting apparently of comminuted vegetable matter, of which considerable
masses are laid in store. Some of the nests are rich in containing many
pounds' weight of this material, while others are apparently quite
destitute of it. There is a soldier form and at least two kinds of workers.
Some species of true ant frequently shares the nest of these white ants,
but on what terms the two kinds of Insects live together is not stated.
TERMITE RAVAGES.—In countries whose climate is favourable to their
constitutions certain kinds of Termites become of great importance to our
own species. Owing to their taste for woody matter and to their habit of
working in concealment, it is no uncommon thing for it to be discovered
that Termites have obtained access to a building and have practically
destroyed the wooden materials used in its construction; all the interior
of the {389}wood being eaten away and only a thin outer shell left intact.
A Termite, _T. tenuis_, was introduced—in what manner is not certainly
known[305]—to the Island of St. Helena, and committed such extensive
ravages there that Jamestown, the capital, was practically destroyed and
new buildings had to be erected. Other such cases are on record.
Destructive species can sometimes be destroyed by placing in the nests a
portion of arsenicated food. This is eaten by some individuals, who perish
in consequence; and their dead bodies being consumed by their comrades, the
colony becomes checked if not exterminated.
The number of described species of Termitidae does not much exceed 100, but
this is certainly only a small portion of those existing, the total of
which may probably reach 1000 species.
Termitidae are classed by some naturalists with the Orthoptera, and they
have a great deal in common with some of the cursorial division of that
Order, more particularly Forficulidae and Blattidae; but they differ from
Orthoptera in the nature and form of the wings. They are also classed by
some, with a few other forms, as a separate Order of Pseudo-Neuroptera
called Corrodentia, but this is not a very satisfactory course, as the
Termitidae do not agree closely with the forms associated with them, while
the aggregate so formed is far from being very distinct from other forms of
Neuroptera. On the whole the best plan appears to be to treat the
Termitidae as forming a distinct family of the Order Neuroptera, or to make
it a distinct Order, as proposed by Grassi. Packard now associates Termites
in an Order with the biting-lice, and calls it Platyptera.
FOSSIL TERMITES.—Termitidae were very abundant in Tertiary times, and the
genera appear to have been then much the same as at present. In Mesozoic
strata the remains of true Termitidae apparently exist in the Lias in
Europe, but farther back than this the family has not been satisfactorily
traced. It was formerly supposed that Termitidae existed in the
Carboniferous strata, but this appears to be very doubtful; and the fossil
remains of that epoch, which were presumed to be those of Termites, are now
referred by Scudder and others to the Neuropteroid division of the Order
Palaeodictyoptera, an Order which is formed entirely of Palaeozoic fossil
remains.
{390}CHAPTER XVII
NEUROPTERA _CONTINUED_—PSOCIDAE (BOOK-LICE AND DEATH-WATCHES)—THE FIRST
FAMILY OF AMPHIBIOUS NEUROPTERA (PERLIDAE, STONE-FLIES).
FAM. IV. PSOCIDAE—BOOK-LICE, DEATH-WATCHES.
_Minute Insects with slender, thread-like, or hair-like antennae; four
delicate membranous wings, the front pair of which are the larger; their
neuration is not abundant and is irregular, so that the cells are also
irregularly arranged; the transverse nervules are only one or two in
number.[306] Prothorax very small, in the winged forms quite concealed
between the head and the large mesothorax; this latter closely connected
with, or fused with, the metathorax. Species quite wingless, or with
wings unfitted for flight, exist; in them the prothorax is not so
extremely small, while the mesothorax is smaller than in the winged
forms. Tarsi of two or three segments. Metamorphosis slight, marked
chiefly by the development of wings and ocelli._
[Illustration: FIG. 241.—_Psocus fasciatus_, England. (After M‘Lachlan.)]
The Psocidae are without exception small and soft-bodied Insects, and are
only known to those who are not entomologists by the wingless forms that
run about in uninhabited or quiet apartments, and are called dust-lice or
book-lice. They are perhaps more similar to Termitidae than to any other
Insects, but the two families differ much in the structure of their wings,
and are totally dissimilar in the nature of their lives.
{391}[Illustration: FIG. 242.—Transverse horizontal section of head of
_Psocus_: _f_, fork or pick; _t_, lingua; _mx_, left maxilla; _c_, cardo;
_p_, stipes; _m.m_, muscles; _m.s_, socket of mandible.]
[Illustration: FIG. 243.—A, Front of head of _Psocus heteromorphus_; _cl_,
post-clypeus; _g_, epicranium: B, transverse horizontal section of
post-clypeus of _Psocus_: _cl_, post-clypeus; _c.m_, clypeal muscles; _g_,
epicranium; _t_, tendons; _l.m_, labial muscle in section; _oe_,
oesophagus; _oe.b_, oesophageal bone. (After Burgess and Bertkau.)]
The antennae consist of eleven to twenty-five joints, or even more, about
thirteen being the usual number; the basal two are thicker than the others,
and are destitute of setae or pubescence such as the others possess. The
maxillae and labium are remarkable. The former possesses a peculiar hard
pick or elongate rod; this is considered by many naturalists to be the
inner lobe, but Burgess thinks it more probably an independent organ,[307]
as it has no articulation of any kind with the outer lobe. The latter is
remarkably thick and fleshy; the palpus is 5-jointed. Other authorities
consider the pick to be certainly the inner lobe; if it be not, the latter
is quite wanting. Hagen agrees with Burgess in stating that the pick slides
in the outer lobe as in a sheath. The labium has a large mentum and a
ligula divided anteriorly into two lobes; at each outer angle in front
there is a globular projection, which is doubtless the labial palpus;
reposing on the labium there is a large free lingua. The labrum is large,
attached to a distinct clypeus, behind which there is a remarkable
post-clypeus, which is usually prominent as if inflated; to its inner face
are attached several muscles which converge to be inserted on a plate
placed below the anterior part of the oesophagus, and called by Burgess the
{392}oesophageal bone; under or within the lingua there is a pair of
lingual glands. Judging from Grosse's study of the mouth of Mallophaga, we
may conclude that the oesophageal bone will prove to be a sclerite of the
hypopharynx. The eyes of the winged forms are frequently remarkably convex,
and there are also three ocelli, triangularly placed on the vertex. The
head is free and very mobile. The coxae are rather small, exserted,
contiguous; the sterna small. The abdomen has usually ten segments, though
sometimes only nine can be detected.
The thorax in Psocidae usually looks as if it consisted of only two
segments. This is due to two opposite conditions: (1) that in the winged
forms the prothorax is reduced to a plate concealed in the fissure between
the head and the mesothorax bearing the first pair of wings; (2) that in
the wingless forms (Fig. 247), though the prothorax is distinct, the meso-
and metathorax are fused into one segment.
[Illustration: FIG. 244.—Reproductive organs of _Clothilla pulsatoria_. A,
Male; _a_, vesiculae seminales; _b_, testes; _c_, vasa deferentia; _d_,
ejaculatory duct. B, Female; _a_, _b_, egg-tubes; _c_, oviduct; _d_,
uterus, containing egg; _e_, accessory gland (the enveloping sac in
section); _f_, its duct. (After Nitzsch.)]
The internal anatomy is only very incompletely known. Nitzsch[308] has,
however, described the alimentary canal and the reproductive organs of
_Clothilla pulsatoria_. The former is remarkably simple: no proventriculus
or crop was found; the stomach is very elongate, and consists of a sac-like
anterior portion and an elongate, tubular posterior part. There are four
Malpighian tubes. The posterior part of the canal is remarkably short, the
small intestine being scarcely as long as the rectum. The ovaries (Fig.
244, B) consist of five egg-tubes on each side; connected with the oviduct
there is a peculiar accessory gland consisting of a sac containing other
small sacs each {393}with an elongate efferent duct; the number of the
secondary sacs varies from one to four according to the individual. The
testis (Fig. 244, A, _b_) is a simple capsule; connected with the base of
the ejaculatory duct there is a pair of elongate accessory glands or
vesiculae seminales.
The life-history has never been satisfactorily sketched. The young greatly
resemble the old, but have no ocelli or wings, and sometimes the tarsi are
of two joints, while in the adult they have three. The antennae have also
in these cases a less number of joints in the young stage. The food is
animal or vegetable refuse substances; many live on fungoid matter of
various kinds, mouldy chaff being, it is said, a favourite pabulum; the
mould on palings is a source of food to many; others live on the rust-fungi
of leaves, and many frequent the bark of trees. They are able to spin webs,
probably by the aid of the lingual glands; the eggs are deposited, in some
cases, on leaves and covered with a web. Hagen says that a peculiar organ,
possibly a gland—he calls it a hose[309]—exists at the base of the tarsal
claws. In our climate most of the species pass the winter in the egg-state.
There may be two generations in a year, perhaps more.
The nomenclature of the wing-veins of Psocidae has given rise to much
discussion.[310] The system shown in the accompanying figure is probably
the most convenient; the subcostal vein (2) is always obscure, and
sometimes can only be detected by very minute examination. Some interesting
information as to the minute structure and mode of formation of the wings
and their nervures has been given by Hagen.[311]
[Illustration: FIG. 245.—Anterior wing of _Elipsocus brevistylus_. (After
Reuter.) 1, Costal vein; 2, subcostal; 3, radial; 4, cubitus; 4_a_,
branches of cubitus; 5, sector of the radius; 5_a_, forks thereof.]
In the young the wings first appear as buds, or outgrowths of the sides of
the meso- and meta-thorax; afterwards the prothorax decreases, while the
other two thoracic segments and the wing-rudiments attached to them
increase. The wings from their very origin appear to be different from
those of the Orthoptera, and the changes that take place in the thoracic
{394}segments in the course of the development, differ from those that
occur in Orthoptera.
[Illustration: FIG. 246.—Micropterous form of _Mesopsocus unipunctatus_.
_a_, _a_, Wings. (After Bertkau.)]
There are several peculiarities connected with the wings. Frequently they
exist, though of no use for flight; some Psocidae that have
perfectly-formed wings are so reluctant to use them that, M‘Lachlan says,
they will allow themselves to be crushed without seeking to escape by
flight. At certain periods, however, some Psocidae float on the wing in
considerable numbers, especially in a moist still atmosphere, and then
drift about like the winged Aphididae, which are frequently found with
them. There is evidence that individuals, or generations, of some of the
winged species occur with only rudimentary wings; although this has been
denied by Kolbe, there can be no doubt about it. The form figured above
(Fig. 246) was described by Bertkau[312] as a distinct genus, but was
afterwards recognised by him[313] to be a short-winged form of _Mesopsocus
unipunctatus_. It is probable that the adult female of this species has the
wings always micropterous, while the male has these organs of the full
size. In other species the condition of the rudimentary wings seems to be
quite constant. The facts concerning the wings of Psocidae are so peculiar
that Kolbe came to the conclusion that the organs exist not because they
are of use for flight, so much as because it is the nature of an Insect to
develop wings.[314]
Some of the species of Psocidae have never any trace of wings. These
apterous forms are mostly included in the division Atropinae, and are
usually very minute; it has been again and again erroneously stated that
they are the young state of winged forms. Hagen kept a large colony of
_Atropos divinatoria_ for some years in confinement, so that he saw
numerous generations as well as many specimens. He found the apterous
condition quite constant.
{395}The association of ocelli with wings is nearly constant in Psocidae.
The genus _Clothilla_—allied to _Atropos_—possesses very rudimentary wings
but no ocelli. Hagen, however, found[315] that in a certain locality no
less than 12 per cent of the individuals of this species were provided with
ocelli,—a most extraordinary variation.
In some of these apterous forms there is found on each side of the
prothorax a tubercular prominence which, according to Hagen, can be
considered only as the rudiment of a wing that never develops. Though no
existing Insect is known to possess rudimentary wings on the prothorax, we
have previously mentioned (p. 344) that in the Carboniferous epoch
appendages of the nature alluded to were not very rare.
A genus of living forms—_Hyperetes_—in which the three thoracic segments
are well developed, but in which there are no alar appendages or rudiments,
is considered by Hagen to be more primitive than the Psocidae found in
amber to which we shall subsequently allude.
The number of described species of Psocidae does not reach two hundred; we
have, however, thirty species or more in Britain.[316] Nietner observed
about the same number in the immediate vicinity of his house in Ceylon. The
isolated and remote Hawaiian group of islands is remarkably rich in
Psocidae. Two thousand is a moderate estimate of the number of existing
species. The largest forms yet discovered belong to the Brazilian genus
_Thyrsophorus_; they attain, however, a breadth of only about one inch with
the wings fully expanded. The Cuban genus _Embidopsocus_ is said to be of
great interest from its approximation to Embiidae. It is at present very
inadequately known.
One (or more) very minute Insects of this family—_Clothilla pulsatoria_
according to Hagen, _Atropos[317] divinatoria_ according to some other
authors—is widely known under the name of the death-watch, owing to its
being believed to make a peculiar {396}ticking noise, supposed to be
prophetic of the decease of some individual—a human being we fancy, not a
death-watch. It is difficult to believe that so minute and soft an Insect
can produce a sound audible to human ears, and many entomologists are of
opinion that the sound in question is really produced by a beetle—of the
genus _Anobium_—which lives in wood, and that as the beetle may be
concealed in a hole, while the _Clothilla_ is seen running about, the sound
is naturally, though erroneously, attributed to the latter. But the rapping
of the _Anobium_ is well known, is produced while the Insect is at large,
and is said to be a different noise from that of the Psocid; evidence too
has been given as to the production of the sound in a workbox when the
Psocid was certainly present, and the most careful search failed to reveal
any beetle.
[Illustration: FIG. 247.—A, _Atropos divinatoria_; B, _Clothilla
pulsatoria_. (After M‘Lachlan.)]
The Rev. W. Derham, who two hundred years ago was Rector of Upminster, in
Essex, and was well known as a distinguished writer and philosopher, gave
an account of the ticking of death-watches to the Royal Society.[318] This
gentleman was a most accurate and minute observer; he was well acquainted
with the ticking of the greater death-watch—_Anobium_—which he describes
very accurately, as well as the acts accompanying it, the details he
mentions being exactly such as occur at the present time. He not only heard
the ticking of the Psocid or lesser death-watch, but repeatedly witnessed
it. He says: "I am now so used to, and skilful in the matter as to be able
to see, and show them, beating almost when I please, by having a paper with
some of them in it conveniently placed and imitating their pulsation, which
they will readily answer." He also states that he could only hear them
beating when it was done on paper, and that this death-watch will tick for
some hours together without intermission, with intervals between each beat,
so that it much resembles the ticking of a watch. The act of ticking was
{397}accompanied by rapping the front of the head on the paper, but Mr.
Derham could not be sure that the sound was produced in that manner,
because each stroke was also accompanied by a peculiar shudder, or recoil.
After a prolonged ticking he observed that another individual of the other
sex made its appearance. The species figured by Mr. Derham more resembles a
_Hyperetes_ than it does either of our two known book-lice, _Atropos_ and
_Clothilla_.
[Illustration: FIG. 248.—The lesser death-watch of Upminster. (After
Derham.) A, magnified; B, natural size.]
[Illustration: FIG. 249.—_Sphaeropsocus kunowii._ From amber. × 30. (After
Hagen.)]
Numerous species of Psocidae are preserved in amber; Hagen[319] has made a
careful study, based on a considerable number of specimens, of about
thirteen such species. They belong to no less than nine genera and five
sub-families. _Sphaeropsocus_ is the most remarkable; this Insect has a
well-developed prothorax, as is the case in the wingless Psocids, and a
pair of large wings or tegmina meeting by a straight suture along the back,
as is usual in beetles, though quite unknown in existing Psocidae. Another
species, _Amphientomum paradoxum_, has the body and appendages covered with
scales like a butterfly or moth; other species, found in gum-copal or still
living, have scales on various parts of the body, but not to so great an
extent as this amber species. The genus _Amphientomum_ is still represented
in Ceylon and elsewhere by living forms; Packard has figured some of the
scales;[320] they appear to be extremely similar to those of Lepidoptera or
Thysanura. The facts connected with this fauna of amber Psocidae would seem
to show that the family was formerly more extensive and important than it
is at present; we should therefore expect to find numerous fossil forms in
strata of date {398}anterior to that of the amber; but this is not the
case, all that is known as to fossil Psocidae being that Scudder has
recently ascribed traces of an Insect found in the Tertiary rocks of Utah
to this family as a distinct genus.
FAM. V. PERLIDAE.
_Insects of moderate or large size, furnished with four membranous wings;
these are usually complexly reticulate; the hind pair are much the
larger, and have a large anal area of more simple venation, which becomes
plicate when folded. The coxae are small, the legs widely separated. The
larvae are aquatic in habits; the metamorphosis is slight._
[Illustration: FIG. 250.—_Pteronarcys frigida_, male. (After Gerstaecker.)]
The Perlidae form a small family of Insects unattractive in their general
appearance. The life-history of each individual consists of two abruptly
contrasted portions; the earlier stage being entirely aquatic, the later
aerial. Hence the Perlidae come into the amphibious division of Neuroptera.
The definition we have given above would, except as regards the texture of
the front wings and the aquatic habits of the larvae, apply to many Insects
of the Order Orthoptera. The Phryganeidae, another {399}family of
Neuroptera, have aquatic larvae and wings somewhat similar in form to those
of the Perlidae, but the members of the two families cannot be confounded,
as the Phryganeidae have hairy front wings and large and contiguous coxae.
The antennae of the Perlidae are long, very flexible, and composed of a
very large number of joints. The parts of the mouth vary a good deal. The
mandibles and maxillae are usually rather small, and all the parts of the
mouth are of feeble consistence or even membranous; the maxillary palpi
are, however, well developed and exserted from the mouth, five-jointed. The
labium is short and but little conspicuous. The mandibles in some forms are
almost membranous, but in other genera they are firmer and are toothed. The
labium is composed of a very large mentum, beyond which is a large piece,
usually undivided, bearing the four terminal lobes; the three-jointed
palpus is seated on the side of the large middle sclerite, which is no
doubt of composite nature. Considerable variety as to the lower lip
prevails. The head is broad and flat; there is an indistinctly-indicated
clypeus, three—more rarely two—ocelli, and on each side an eye neither very
large nor perfect. The prothorax is free, and has a flat, margined notum.
The meso- and the meta-thorax are large, equal segments. The pro-, meso-,
and meta-sternum are large pieces; between the first and second, and
between the second and third there is an intervening membrane. The
metasternum is much prolonged backwards, and has on each side a peculiar
slit; similar orifices exist on the other sterna (Fig. 254, _o_). Newport,
who has examined them in _Pteronarcys_, says that they are blind
invaginations of the integument; he calls them the sternal or furcal
orifices.[321] According to this naturalist these very peculiar openings
pass into the body "as strong bone-like tubes, diverging from the axis to
the periphery of the body in the immediate vicinity of some of the
principal tracheae, but that they do not in any way communicate with them,
as they terminate abruptly as caecal structures." He thinks them analogous
with the endo-skeleton of other Insects; a view which cannot be considered
sufficiently established. Laboulbène states[322] that when _Perla parisina_
is seized and placed on its back, it does not move, but emits a liquid at
the base of the articulation of the legs. {400}This suggests that it may
come from these sternal orifices. The abdomen consists of ten dorsal
plates, the first being short, and of nine ventral; the dorsal plates are
much more ample transversely than the ventral. Frequently the hind body is
terminated by two long, many-jointed cerci, looking like antennae. The
coxae are small, not prominent, and are directed outwards. The legs are
slender, the tibiae often grooved. The tarsi are three-jointed, terminating
in two claws and a more or less distinct pad. In the genus _Isopteryx_ an
auditory organ has been described as existing in the legs, in a position
similar to that of the analogous structures in Termitidae and Blattidae.
The wings when closed repose flat on the back, and fold and overlap so that
only one is seen (Fig. 251); in this state the costal portion of each front
wing is turned downwards, so as to protect to some extent, the sides of the
body.
[Illustration: FIG. 251.—_Perla maxima._ (After Pictet.)]
[Illustration: FIG. 252.—_Perla_ sp., nymph, showing tracheal gills.
Pyrénées orientales.]
The early stages are known, but have not been described minutely, and there
appears to be very little information as to the youngest life. All the
species are, when immature, aquatic in their habits; the larvae greatly
resemble the perfect Insects in form, though differing in not possessing
wings and in the ocelli being merely opaque spaces. They have rather large
compound eyes; the future wings are represented by lobe-like
prolongations—varying in length according to age—of the meso- and
meta-notum. In the Nemourae the cerci are absent in the imago though
present in the young. The larvae of Perlidae are carnivorous {401}and are
able to swim well, the legs being provided with abundant swimming hairs;
they, however, as a rule, prefer to walk at the bottom of the pool, or on
rocks or boulders in the water they live in.
One of the most peculiar features of the Perlidae is their respiratory
system. Unfortunately the greatest differences of opinion have prevailed on
various matters in connexion with this subject, and there are several
points about which it is not possible at present to express a decided
opinion.
[Illustration: FIG. 253.—Tracheal gill and portion of a trachea of
_Pteronarcys_. (After Newport.)]
The larvae have no stigmata; it appears to be generally agreed that there
is in them no means of admitting air to the tracheal system by means of
orifices. Some breathe entirely through the integument, the process being
aided by the accumulation of tracheae at the spots where the breathing
orifices should be, and where the integument is more delicate. Others,
however, possess gills in the form of protruded bunches of filaments,
connected with tracheae in the manner shown in Fig. 253. These filamentous
branchiae occur in numerous species of the family, and are situate on
various parts of the body, but many species are destitute of them in
genera, other members of which possess the filaments. In some Nemourae
instead of bunches of filaments there are tubular projections on the
prothoracic segment; and in _Dictyopteryx signata_ similar structures occur
even in the cephalic region, Hagen stating[323] that there exists a pair on
the submentum and another on the membrane between the head and the thorax.
In the imago state, stigmata are present in the normal fashion, there being
two thoracic and six abdominal pairs. In several species the filaments
persist in the imago, so that in these cases we meet with the curious
condition of the coexistence of branchiae with a well-developed and
functionally active system of spiracles; this is the more curious because
the creatures usually have then nothing to do with the water, it having
been ascertained that in these cases the species live out of the water as
other terrestrial and aerial Insects do. These instances of persistence
{402}of branchiae during the aerial life have been the source of some
perplexity; the condition was shown to exist in _Pteronarcys_ by Newport,
and has since been demonstrated in various other forms. Newport believed
that the imago of _Pteronarcys_ breathes by means of the gills, although it
lives out of the water and possesses spiracles; and he informs us that Mr.
Barnston observed the Insect when on the wing "constantly dipping on the
surface of the water." Hence Newport concluded that _Pteronarcys_ in the
winged state is "an amphibious animal." That a winged Insect should live in
the air and yet breathe by means of gills would be truly extraordinary, and
there can be little doubt that Newport's idea was erroneous. Hagen[324] was
able to examine living imagos of the species in question. He found that
they avoided the water, and though he placed some individuals therein, yet
they did not use the gills. He also informs us that the branchiae have,
during life, a shrivelled appearance, indicating that they are not
functionally active, but are merely useless organs carried over to the
imago from the previous instar, in which they were truly the means of
obtaining air. Hagen also ascertained that the spiracles of the imago are
in a normal state, being adapted for breathing, even as far back as the
seventh abdominal segment.
[Illustration: FIG. 254.—Under side of body of _Pteronarcys regalis_,
imago. (After Newport.) _g_, Tracheal gills; _o_, sternal orifices.]
Great difference of opinion has prevailed as to the relations of the
branchiae to the stigmata, it having been contended that the falling off of
some of the branchiae left the stigmatic orifices. The facts appear to be
only consistent with the conclusion that the two are totally independent
organs. This subject has been investigated by Palmén,[325] who finds that
in Perlidae—contrary to what occurs in may-flies—the species are either
entirely destitute of gills, or these organs are persistent throughout
life. It is not to be inferred from this that the gills in the
{403}perennibranchiate Perlidae are as conspicuous as they are in the
exceptional _Pteronarcys_: for it appears that at the final moult the gills
usually become very much contracted and concealed by the new integument; in
some cases they merely appear as slight prominences in the neighbourhood of
the stigmata.
Pictet, Dufour, Newport, and Imhof[326] have studied the internal anatomy.
The alimentary canal is remarkable for the enormous oesophagus; there is no
distinction between this and the crop. A proventriculus is quite absent,
and there are no chitinous folds in the position it usually occupies. The
true stomach is small, and only commences in the fourth abdominal segment.
It has a prolonged lobe on each side in front, and in addition to this
eight sacs; thus there are formed ten diverticula, fastened to the
posterior part of the oesophagus by ligaments. The terminal portion of the
stomach is small, and apparently only distinguished from the short
intestine by the point of insertion of the Malpighian tubes; these vary in
number from about twenty to sixty. There are two pairs of large salivary
glands. In _Pteronarcys_ the caecal diverticula of the stomach are wanting.
In some Perlidae the terminal parts of the gut are more complex than in
_Perla maxima_; Newport figures both an ilium and colon very strongly
differentiated, and states that these parts differ much in _Perla_ and
_Pteronarcys_. According to him the stomach is embraced by a network of
tracheae, and Imhof tells us that he found the stomach to contain only air.
[Illustration: FIG. 255.—Alimentary canal and outline of body of _Perla
maxima_. (After Imhof.) _l_, Upper lip; _mh_, buccal cavity; _ap_, common
termination of salivary ducts; _o_, oesophagus; _s_, salivary glands; _ag_,
duct of salivary gland; _b_, anterior diverticula of stomach; _lg_, their
ligaments of attachment; _mp_, Malpighian tubes; _r_, rectum; _af_, anal
orifice.]
The brain is small, but, according to Imhof, consists of four amalgamated
divisions; the infra-oesophageal ganglion is small, {404}and placed very
near the brain. There are three thoracic and six abdominal ganglia on the
ventral chain. The nerves to the wings are connected with the longitudinal
commissures of the ventral chain by peculiar, obliquely-placed, short
commissures. The reproductive glands are peculiar, inasmuch as in each sex
the pair of principal glands is connected together in the middle. The
testes thus form an arch consisting of a large number of sub-spherical or
pear-shaped follicles; the vasa deferentia are short in _Perla maxima_, and
there are no vesiculae seminales; the ejaculatory duct is divided into
three parts by constrictions. In _Pteronarcys_ and in _Perla bicaudata_,
according to Newport and Dufour, the vasa deferentia are very long and
tortuous, and there are elongate vesiculae seminales. The arrangement of
the extremely numerous egg-tubes is analogous to that of the follicles of
the testes, so that, as Dufour says, there is but a single ovary; connected
with the short, unpaired portion of the oviduct, there is a large
receptaculum seminis, and near the terminal orifice of the duct there is in
_P. maxima_ an eight-lobed accessory gland.
[Illustration: FIG. 256.—The pair of united ovaries of _Perla maxima_: _o_,
egg-tubes; _ov_, oviduct; _r_, receptaculum seminis concealing the orifice
of the duct and an accessory gland.]
[Illustration: FIG. 257.—Egg of _Perla maxima_. (After Imhof.) _c_,
chorion; _d_, oolemn; _gs_, glass-like covering of micropyle apparatus;
_l_, cavity under same; _g_, canals penetrating chorion.]
The eggs are produced by Perlidae in enormous numbers: they are rather
small, but peculiar in form, and possess at one extremity a micropyle
apparatus, covered by a glassy substance through which Imhof could find no
orifice. On the other hand, the chorion on another part of the egg is
perforated by several canals.
{405}The Perlidae being of aquatic habits in their early stages, and,
notwithstanding their ample wings, very poor adepts in the art of flying,
are rarely found at any considerable distance from their native element.
They are specially fond of running water, and delight in the neighbourhood
of waterfalls, or other spots where the current is broken by obstacles so
that a foaming water results. It is probable that the larvae which breathe
by means of gills find an advantage in living in strongly-aerated water.
Mountain streams and torrents are therefore specially affected by them; but
Pictet informs us that they do not like the waters descending from
glaciers. The food of the larvae is believed to be chiefly young may-flies,
or other small, soft creatures, and it may possibly be owing to the absence
of these that the Perlidae do not affect the glacier streams. Although
Perlidae are remarkable for their capacity for enduring cold, it is
possible that they may require warmth of the water at some period of their
development, and this the glacier-streams cannot offer to them. They are
among the earliest Insects to appear in the spring in Europe. Mr. Barnston
says that on the Albany river in Canada the nymph of _Capnia vernalis_
comes up frequently in the cracks of the ice and casts its skin there; "it
frequently comes up when the thermometer stands at freezing." Of _Nemoura
glacialis_, which inhabits similar localities, he says that "it appears in
the spring (end of March or beginning of April) when the ice becomes
honeycombed, and even before then, at the same time as _Capnia vernalis_.
It pairs in the crevices of decaying ice. The male has long antennae, and
his wings are generally rumpled as if glued together." Newport entertained
the idea that those Perlidae that live at low temperatures are of lower
organisation than the other forms of the family.
It is a remarkable fact that several Perlidae frequently have—like _Nemoura
glacialis_—the wings of the male much reduced in size; this being the
contrary of the rule that usually prevails among Insects to the effect
that, when there is a difference in the powers of flight, or even in the
size of the wings, it is the male that is superior. Mr. J. J. Lister met
with a very interesting Perlid at Loch Tanna in Arran at the beginning of
April 1892. In this Insect, which is, according to Mr. M‘Lachlan, a form of
_Isogenus nubecula_, the wings of the female (Fig. 258, B) are reduced to a
size much less than those of ordinary {406}Perlidae, while those of the
male (Fig. 258, A) are mere useless rudiments. Morton has pointed out that
in Scotland more than one species of _Taeniopteryx_ occasionally produces
micropterous males, and he associates this phenomenon with the early time
of their appearance "almost in winter."[327] In _Nemoura trifasciata_ this
reduction of the wings takes another but equally curious form; the hind
wings of the male being long enough to cover the body, while the anterior
pair are reduced to mere rudiments.
[Illustration: FIG. 258.—_Isogenus nubecula_, Loch Tanna. A, Male; A',
wings of male more magnified; B, wings of female.]
The phenomena of micropterism in Perlidae are well worthy of more detailed
investigation. Mr. Morton informs the writer that the male of _Perla
maxima_ (Fig. 251) in North Britain has the wings so short that they cannot
be of any use as organs of flight. In Central Europe the wings are ample,
as shown in our figure. In _Perla cephalotes_ the male is short-winged in
both Britain and Central Europe; of the male of _Dictyopteryx microcephala_
only the micropterous form is known to exist. In _Isogenus nubecula_ (Fig.
258) it appears that the wings of the female are always more ample than
those of the male of the same locality, and that local micropterism affects
the two sexes unequally. Within the Arctic circle this Insect is usually of
the Scotch form, though the male there occasionally has more ample wings.
It has been observed that in some Perlidae the eggs, after they have been
extruded, are carried about by the female; for what reason is not at all
known. They are said to be enclosed in a membranous capsule at the apex of
the abdomen. The number of eggs deposited is sometimes very large,
amounting to five or six thousand, and they are often of very minute size.
About twenty-four species of Perlidae occur in Britain.[328] The
{407}species from all parts of the world existing in collections probably
scarcely exceed two hundred. The insignificance of this number is no doubt
chiefly due to the fact that these unattractive Insects are rarely captured
by collectors, and are so fragile that unless good care is taken of them,
specimens soon go to destruction after being dried. Perlidae are known to
occur in most parts of the world, so that the number of species really
existing may reach two or three thousand. They are known to anglers as
stone-flies and creepers and are a favourite bait for trout.
The family in its character comes near to the Orthoptera, especially to the
more simple forms of Phasmidae, but the two groups differ in the texture of
the front wings and in the structure of the mouth-parts, as well as in the
different proportions of the mesothorax and metathorax. According to
Pictet, in the Australian genus _Eusthenia_ the trophi (Fig. 259) approach
nearer to those of the Orthoptera, so that it appears possible that a more
intimate connexion will be found to exist as more forms are discovered. Of
the groups we include in Neuroptera, Perlidae are in structure most allied
to Sialidae, but the development in the two groups exhibits very important
distinctions. Brauer treats the Perlidae as forming a distinct Order called
Plecoptera, a name applied to the family by Burmeister many years ago.
[Illustration: FIG. 259.—A, Maxilla, B, labium of _Eusthenia spectabilis_.
(After Pictet.)]
Several species of Perlidae, considered to belong to existing genera, have
been found in amber. A fossil from the Eocene deposits in the Isle of Wight
and another from the Miocene of Continental Europe are referred to the
family. Brauer has recently described[329] some fossils from the Jurassic
formation in East Siberia as forming three genera, now extinct, of
Perlidae.
Brongniart informs us[330] that several fossils have been found {408}in the
Carboniferous strata of Commentry that justify us in asserting that allies
of Perlidae then existed. He considers these Carboniferous Insects to have
belonged to a separate family, Protoperlides. The fragments are, however,
so small that we must await further information before forming a definite
opinion as to these Protoperlides.
{409}CHAPTER XVIII
AMPHIBIOUS NEUROPTERA _CONTINUED_—ODONATA, DRAGON-FLIES
FAM. VI. ODONATA—DRAGON-FLIES.
(LIBELLULIDAE OF SOME AUTHORS)
_Elongate Insects with very mobile head and large eyes, with small and
inconspicuous antennae ending in a bristle; with four elongate wings
sub-equal in size and similar in texture, of papyraceous consistency and
having many veinlets, so that there exists a large number of small cells.
All the legs placed more anteriorly than the wings. The earlier stages of
the life are aquatic; there is great change in the appearance of the
individual at the final ecdysis, but there is no pupal instar._
The dragon-flies form a very natural and distinct group of Insects. All the
species are recognised with ease as belonging to the family. They are
invariably provided with wings in the perfect state, and many of them are
amongst the most active of Insects. Their anatomy is, in several respects,
very remarkable.
The head is large and is concave behind; it is attached to the thorax in
such a way that it rotates on two cervical sclerites that project forwards,
and in some cases almost meet in a point in front; hence it possesses
extreme mobility, the power of rotation being very great.
The eyes are always large; in some cases they are even enormous, and occupy
the larger part of the area of the head: the upper facets of the eye are in
many cases larger than the lower, and in a few forms the line of division
is sharply marked transversely. There are three ocelli, which, when the
size of the compound eyes is not too great, are placed in the usual
{410}manner as a triangle on the vertex; but in the forms where the
compound eyes are very large the portion of the head between is, as it
were, puffed out so as to form a projection just in front of where the eyes
meet, and one ocellus is then placed on each side of this projection, an
antenna being inserted quite close to it; the third ocellus is placed in
front of the projection we have mentioned, by which it is often much
concealed; this anterior ocellus is in some cases of unusually large size,
and oval or transverse in form.
[Illustration: FIG. 260.—_Anax formosus_, Britain. (After Migneaux.) (The
legs are not in a natural position.)]
The parts of the mouth are very peculiar, especially the lower lip: we will
briefly allude to its characters in the highly modified forms, premising
that in the smaller and less active species it is less remarkable. The
Libellulidae are carnivorous, their prey being living Insects which are
captured by the dragon-fly on the wing; it is believed that the mouth is
largely instrumental in the capture, though the flight of these Insects is
so excessively rapid that it is difficult, if not impossible, to verify the
action of the mouthpieces by actual observation.
{411}[Illustration: FIG. 261.—A, Maxilla of _Libellula quadrimaculata_; B,
labium of _Aeschna grandis_. _p_, _p′_, Palpus; _a_, terminal spine of
palpus; _c_, cardo; _t_, stipes; _s_, squama; _le_, outer lobe of maxilla,
partly covered by, _li_, inner lobe; _m_, mentum; _r_, intervening lobe.
(After Gerstaecker.)]
For the purpose of securing the prey a mouth that can change its capacity
to a considerable extent and with rapidity is a desideratum, and these
qualities are present in the mouths of those Libellulidae that capture
their prey while hawking. The upper lip is very mobile, is pendent, and
closes the mouth above, while the lower lip entirely closes the under part
by means of two mobile plates; these in some forms (_Libellula_) meet
together in the mesial line, while in others a third plate separates them
in the middle (Fig. 261, B, _li_). These plates are, according to
Gerstaecker's view,[331] portions of the much changed labial palpi, the
part that separates them in _Aeschna_ being the inner lobes of the labial
maxillae; in _Libellula_, where the dilated and valve-like joints of the
palpi meet in the middle line, the labial lobes remain small and are
overlapped by the dilated portions of the palpi. The maxillae proper (Fig.
261, A) are less peculiar, their chief character being that the inner and
outer lobes are not separated, and that the palpus is of only one joint.
Some entomologists take, however, another view of this structure, looking
on the palp-like outer part (_p_ of our figure) as the true outer lobe of
the maxillae, the palpus proper being in that case considered to be
entirely absent. The mandibles are very powerful, and armed with largely
developed teeth. In the interior of the mouth there is a large, free,
semi-membranous lingua, the posterior part of its delicate inferior lamina
being connected with the mentum; the upper lamina of the lingua is stronger
and is pilose. The antennae of the dragon-flies are always small, and
consist of two stouter joints at the {412}base, and a terminal part which
is very slender and pointed, and formed of four or five joints.
The prothorax is always small; the pronotum is distinct, though in some
forms it is quite concealed in the concavity of the back of the head; the
sternum is small; the anatomy of the pleura and basal pieces of the legs is
obscure.
[Illustration: FIG. 262.—A, _Agrion pulchellum_, natural size; B, _Aeschna
cyanea_, profile; C, same from front to show position of legs. ⅔ natural
size.]
The meso- and meta-thorax are very intimately combined, and their relations
are such that the former is placed much above the latter. This peculiarity
is carried to its greatest extent in some of the Agrioninae (Fig. 262, A),
where not only are the wings placed at a considerable distance behind the
three pairs of legs, but also the front pair of wings is placed almost
directly above the hind pair. In the Anisopterides these peculiarities are
much less marked (Fig. 262, B), nevertheless even in them the three pairs
of legs are placed quite in front of the wings. This peculiar structure of
the wing-bearing segments is accompanied by an unusual development of the
pleura, which, indeed, actually form the larger part, if not nearly the
whole, of the front region of the dorsal aspect of these two segments. We
shall not enter into more minute particulars as to the structure of the
thorax, for difference of opinion prevails as to the interpretation of the
parts.[332] The abdomen is remarkable for its elongation; it is never
broad, and in some genera—_Mecistogaster_, _e.g._—it attains a length and
slenderness which are not {413}reached by any other Insects. It consists of
ten segments and a pair of terminal calliper-like or flap-like processes of
very various sizes and forms.
The wings of the dragon-flies are usually transparent and provided with a
multitude of small meshes. The hind wings are about as large as the front
pair, or even a little larger; the main nervures have a sub-parallel
course, and are placed in greater part on the anterior region of each wing.
The relations of the more constant nervures and the cells of which they are
parts form a complex subject, and are amongst the most important of the
characters used in classifying these Insects. The wings are always elongate
in comparison to their breadth and have no folds; they are held partially
extended, or are placed so as to project backwards, or backwards and
outwards. They exhibit another peculiarity, inasmuch as the front or costal
margin is slightly uneven before or near the middle, giving rise to an
appearance such as might result from the breaking and subsequent mending of
the marginal rib at the spot in question, which is called the nodus. In
some forms a peculiar character exists in the shape of a small opaque space
called the membranule, lying close to the body of the Insect in the anal
area of the wing, as shown in Fig. 260.
The legs are slender and are chiefly remarkable for the beautiful series of
hair-like spines with which they are armed, and which in some forms (_e.g._
_Platycnemis_, Fig. 264) are of considerable length. We believe that the
legs are of great importance in capturing the prey, they being held
somewhat in the position shown in Fig. 262, C. The tarsi are three-jointed.
In the male of _Libellago caligata_ the legs exhibit a remarkable
condition, the tibiae being dilated, and on the upper side of a vivid red
colour, while below they are white. This coloration and form are each
unusual in the family. The male of _Platycnemis pennipes_, a British
species (Fig. 264), shows a similar dilatation of the tibiae, but to a less
extent and without any great difference in the colour of the two faces of
the dilatation. This dilatation reaches its maximum in _Psilocnemis
dilatipes_ M‘Lach. The position of the legs in relation to the other parts
of the body is peculiar to the dragon-flies; the legs seem to be unfit for
walking, the Insects never using them for that purpose.
{414}Several peculiarities in the internal anatomy deserve notice. The
alimentary canal in _Libellula_ is about as long as the body, the
oesophagus and chylific stomach being elongate, while the intestine is
short and divided into only two parts; there is no definite proventriculus.
The Malpighian tubules are shorter than usual; they are about forty in
number. The male has no vesiculae seminales; the vasa deferentia are
elongate, and the ejaculatory duct is very short, being in fact merely a
common sinus formed by the terminations of the vasa deferentia. The opening
of this duct is situated on the penultimate ventral plate; the organs of
intromission are, however, placed much anterior to this, on the under side
of the second segment. The mode in which the fertilising fluid is
transferred from the ninth to the second segment is not well understood,
but it is known that the abdomen is flexed by the Insect so as to bring the
ninth ventral plate into contact with the second. The three thoracic
ganglia of the nervous chain are all contiguous, though not completely
amalgamated; the abdominal ganglia are seven in number, and are all
separated, the terminal one being larger than the others. Dufour, after
repeated dissections, was unable to find any salivary glands, but Olga
Poletajewa[333] states that they exist.
The Odonata must be ranked among the most highly-organised Insects so far
as external structure and powers of locomotion are concerned; the peculiar
modifications of the thoracic segments and the relative positions of the
wings and legs mark a great departure from the normal type of Insect
structure. Their prey consists of living Insects, which they capture on the
wing by their own superior powers of flight. They destroy a great many
Insects, their appetite for food being, as in the cases of the Mantidae and
of the tiger-beetles, apparently almost insatiable. They are admirably
constructed for the purposes of their predatory lives; they fly with great
swiftness and change the direction of their flight with admirable facility.
They are, however, dependent on sunshine, and conceal themselves in dull
and cloudy weather. The larger Insects of the family belong to the division
Anisopterides (Fig. 260, _Anax formosus_) and some of these may, in our own
country, usually be seen, in the bright sunshine of the summer and autumn,
engaged in hawking in their favourite haunts. Places where other Insects
{415}abound are naturally those most frequented; the glades of woods,
country lanes and hedge-sides, the borders of streams and the margins of
sheets of water are the places they most affect. They inspire the rustics
with some feeling of fear, and hence have received the name of
"horse-stingers," and in North America are called "devil's
darning-needles." The aversion to dragon-flies may perhaps be due to their
appearance, which is certainly, in the case of some of our species of
_Aeschna_, _Cordulegaster_, and _Gomphus_, very remarkable, consisting of a
dark ground-colour with bars and spots of vivid green or yellow, giving, it
must be admitted, a peculiar, even savage appearance to the Insects.
Whatever the reason may be, they are, it is certain, held in much fear, and
it is difficult to induce a country lad to touch one even when it is
captured and held by another person. The idea of dragon-flies being
dangerous to anything but their Insect victims is, however, entirely
erroneous; they may be captured and handled without their inflicting any
injury. It is probable that the life of the imago may endure for several
weeks if not months. It is known that _Sympycna fusca_—a common European
though not British dragon-fly—hibernates in the imago state.
In the case of the large dragon-flies we have mentioned, each individual
appears to have a domain, as it were, of its own. Westwood tells us that he
has seen what he believed to be the same individual hawking daily for
several weeks together over a small pond. The writer observed a specimen of
_Cordulegaster annulatus_ to frequent a particular bush, to which it
returned—frequently to the same leaf—after an excursion in search of food.
The way in which these Insects actually seize their prey has not yet been
made clear; it is certain that they capture flying Insects, and it seems
most probable, as we have already said, that this is done by means of the
legs. These, as we have said, are inserted so as to be very near to the
mouth; they are directed forwards, and are held bent at right angles so as
to form a sort of net, and are armed with a beautiful system of fine
spines; it is probable that if the dragon-fly pursue an Insect on the wing
and strike it with the trap, formed by its six legs (Fig. 262, C), then
these immediately come together under the mouth, so that the victim,
directly it is captured by the leg-trap of its pursuer, finds itself in the
jaws of its destroyer. It is perhaps impossible to {416}verify this by
actual observation, as the act of capture and transfer is so very brief and
is performed in the midst of a rapid dash of flight, but it seems more
probable that the prey is first struck by the legs than that the mouth is
the primary instrument of capture. The excessive mobility of the head
permits the victim to be instantly secured by the mouth, and the captured
fly is turned about by this and the front pair of legs, and is nipped
rapidly so that the wings and drier parts fall off; the more juicy parts of
the prey are speedily squeezed into a little ball, which is then swallowed,
or perhaps we should rather say that the mouth closes on it, and submits it
to further pressure for the extraction of the juices. We have already noted
that many of these large and active dragon-flies, particularly in the
Libellulinae and Aeschninae, have their eyes distinctly divided into two
parts, the facets in the lower part of the eye being different from those
of the upper part. Exner considers[334] that the upper division is for the
perception of movement, the lower for the perception of the form of resting
objects. Plateau thinks[335] that the dragon-flies perceive only movement,
not form.
[Illustration: FIG. 263.—Inner view of a portion of the left side of body
of _Libellula depressa_, showing a part of the mechanism of flight, viz.
some of the chitinous ridges at base of the upper wing, and some of the
insertions of the tendons of muscles. A, line of section through base of
upper wing, the wing being supposed to be directed backwards; C, upper
portion of mechanism of the lower wing; _b_, lever extending between the
pieces connected with the two wings. (After von Lendenfeld.)]
The splendid acts of flight of the Anisopterid Odonata are accomplished by
the aid of a complex arrangement of chitinous pieces at the bases of the
wings (Fig. 263). In Insects with considerable powers of flight the hind
wings are usually subordinate in functional importance to the anterior, to
which they are attached by a series of hooks, or some other simple
mechanism, on the wings. {417}In the Odonata the two wings of each pair are
quite free, but they are perhaps brought into correlative action by means
of a lever of unusual length existing amongst the chitinous pieces in the
body wall at the base of the wings (Fig. 263, _b_). The wing muscles are
large; according to von Lendenfeld[336] there are three elevator, five
depressor, and one adductor muscles to each wing: he describes the wing
movements as the results of the correlative action of numerous muscles and
ligaments, and of a great number of chitinous pieces connected in a jointed
manner.
Amans[337] has suggested that the mechanism of flight of the dragon-fly
would form a suitable model for a flying-machine, to be propelled by
electricity.
[Illustration: FIG. 264.—_Platycnemis pennipes_, ♂, Britain.]
The Zygopterides—the second of the two divisions of the Odonata—are Insects
different in many respects from the large and robust Anisopterides. The
division comprises the delicate Insects called "demoiselles," damsel-flies,
by the French (Fig. 262, A, and Fig. 264). Great power of flight is not
possessed by these more fragile Insects; they flit about in the most gentle
and airy manner from stem to stem of the aquatic plants and grasses that
flourish in the localities they love. To this group belong the fairy-like
Insects of the genus _Calepteryx_, in which various parts of the body and
wings are suffused with exquisite {418}metallic tints, while sometimes the
two sexes of one species have differently coloured wings. The smallest and
most delicate dragon-flies that are known are found in the tropics; some of
the genera allied to _Agrion_ consist of Insects of extraordinary fragility
and delicacy.
Although the mature Odonata are so pre-eminently endowed for an aerial and
active life, yet in the earlier stages of their existence they are very
different; they are then, without exception, of aquatic habits; though
carnivorous also in this period of their existence, they are sluggish in
movement, lurking in concealment and capturing their prey by means of a
peculiar conformation of the mouth, that we shall subsequently describe.
Their life-histories are only very imperfectly known.
The eggs are deposited either in the water or in the stem of some aquatic
plant, the female Insect occasionally undergoing submersion in order to
accomplish the act. The young on hatching are destitute of any traces of
wings (Fig. 265), and the structure of the thoracic segments is totally
different from what it is in the adult, the rectal respiratory system (Fig.
265, _x_) to which we shall subsequently allude, being, however, already
present. The wings are said to make their first appearance only at the
third or fourth moult. At this time the pleura of the second and third
thoracic segments have grown in a peculiar manner so as to form a lateral
plate (Fig. 266, B, shows this plate at a later stage), and the wing-pads
appear as small projections from the membranes at the upper margins of
these pleural plates (Fig. 266, A, B). The plates increase in size during
the subsequent stadia, and meet over the bases of the wing-pads, which also
become much longer than they were at first. The number of moults that occur
during growth has not been observed in the case of any species, but they
are believed to be numerous. There is no pupa, nor is there any well-marked
quiescent stage preceding the assumption of the winged form at the last
ecdysis, although at the latter part of its life the nymph appears to be
more inactive than usual. When full grown, the nymph is more like the
future perfect Insect than it was at first, and presents the appearance
shown in Figs. 266 and 270. At this stage it crawls out of the water and
clings to some support such as the stem or leaf of an aquatic plant; a few
minutes after doing so the skin of the back of the thoracic region splits,
and the imago emerges from the nymphal skin.
{419}[Illustration: FIG. 265.—Larva of _Diplax_ just hatched. _n_, a
ganglion of the ventral chain; _d_, dorsal vessel; _x_, tracheal network
round rectum. (After Packard, _P. Boston Soc._ xi. 1868, p. 365.)]
[Illustration: FIG. 266.—_Ictinus_ sp., nymph, Himalaya. A, Dorsal, B,
lateral view. (After Cabot.)]
The nymphs never have the body so elongate as the perfect Insect, the
difference in this respect being frequently great, and the nymphs of the
subfamily Libellulinae being very broad (Fig. 266, nymph of _Ictinus_ sp.);
consequently the creature on emergence from the nymph-skin is very much
shorter than it will soon become. Extension begins to take place almost
immediately; it has been thought by some that this is accomplished by
swallowing air; this is, however, uncertain. At first the wings have only
the length of the wing-pads of the nymph, and their apical portion is an
unformed mass. The colour of the perfect Insect is not present when the
emergence takes place. The wing grows quickly until the full length is
attained. In the genus _Agrion_ {420}the expansion of the wings is
accompanied by frequent elevations and depressions of the body, and
occupies an hour or so; the elongation of the abdomen is not so soon
completed, and its brilliant colours do not appear for several hours.
[Illustration: FIG. 267.—Young nymph of _Aeschna_ sp. (Cambridge) about
fourth moult.]
The mouth of the nymph bears a remarkable structure called the mask (Fig.
268). It is apparently formed by a backward growth of the bases of the
labium and lingua, a hinge being formed between the two at the most
posterior point of their growth. The prolonged portions of these parts are
free: usually the mask is folded under the head, but it can be unfolded and
thrust forward, remaining then attached to the head by means of the more
anterior parts of the lingua and by the maxillae, the whole of the elongate
apparatus being, when used, extended from this anterior part of its
attachment. The front parts of the labium form a prehensile apparatus armed
with sharp teeth, so that the structures make altogether a very effectual
trap, that can be extended in order to secure the prey.
[Illustration: FIG. 268.—Under side of head of _Calepteryx virgo_, nymph,
with mask unfolded. _a_, Lingua; _b_, line from which the mask swings; _c_,
line of doubling up; _d_, lower lip proper; _e_, articulated lateral
processes thereof.]
The fact that the dragon-fly passes suddenly, in the middle of its
existence, from an aquatic to an aerial life, makes the condition of its
respiratory organs a subject for inquiry of more than usual interest.
Réaumur was of opinion that the nymph was, in spite of its aquatic
existence, provided with an extensive system of stigmata or orifices for
breathing air; this was, however, denied by Dufour, and his opinion seemed
to be supported by the fact that other means of obtaining air were
discovered to exist in these nymphs. The inquiries connected with the
respiration {421}of Odonata are still very incomplete, but some interesting
points have been ascertained, the most important of which is perhaps the
existence in some forms of a respiratory system in connexion with the
posterior part of the alimentary canal (Fig. 269). In the nymphs of
Anisopterides the system consists of four main tracheal trunks, traversing
the length of the body, and by their ramifications and inosculations
forming an extensive apparatus. Connected with the four main trunks we have
described, there is a shorter pair confined to the abdomen, where it
supplies a large number of branches to the walls of the stomach. The dorsal
pair of the main tubes give numerous subsidiary branches to the outside of
the rectum, and the ventral pair furnish a smaller number. The walls of the
gut are penetrated by the branches, which inside the rectum form numerous
loops; these, covered by a membrane, project into the interior in the form
of multitudinous papillae (Æschninae). In the Libellulinae the papillae are
replaced by more flattened processes or lamellae. The structures attain a
remarkable development, there being in _Aeschna cyanea_ upwards of 24,000
papillae.
[Illustration: FIG. 269.—Portion of tracheal system of nymph of _Aeschna
cyanea_. _R_, _R_, _R_, _R_, rectum; _A_, anus; _td_, dorsal; _tv_,
ventral, tracheal tubes; _M_, Malpighian tubes. (After Oustalet.)]
These rectal gills obtain air from water admitted into the rectum for the
purpose; the extremity of the body being armed with projections of variable
form, that can be separated to allow ingress and egress of the fluid, or
brought into close apposition so as to close the orifice. The water so
taken in can, by some species, be ejected with force, and is used
occasionally as a means of locomotion. These rectal branchiae can absorb
free air, as well as air dissolved in water. If the fluid in which the
creatures are placed has been previously boiled, so as to expel the air
from it, the nymphs then thrust the {422}extremity of the body out of the
water and so obtain a supply of air.
Oustalet and Palmén state that at the last ecdysis the lamellae, or the
papillae, do not disappear, but remain quite empty, and are consequently
functionless, while on the tracheal trunks there are developed air vesicles
to fit the creature for its aerial career. Hagen says that in _Epitheca_
the whole structure of the gills is shed at the final moult.
The subject of the rectal branchiae of _Libellula_ has been discussed and
illustrated by Chun,[338] who states that Leydig has made known that in
_Phryganea grandis_ a structure is found connecting the rectal branchiae of
_Libellula_ with the rectal glands of some other Insects. We have not been
able to find a confirmation of this in the writings of Leydig or elsewhere.
[Illustration: FIG. 270.—_Calepteryx virgo_, mature nymph, Britain.]
In the nymphs of the Zygopterides the highly-developed rectal branchiae
found in the Aeschninae and Libellulinae do not exist, and the respiration
seems to be of a complex character. In one division of the
Zygopterides—Calepteryginae—rectal gills of an imperfect character are said
by Hagen and others to exist.[339] The nymphs of the Zygopterides are
provided with three mobile processes at the extremity of the body (Fig.
270); these serve the purposes of locomotion. They are believed to possess
also a respiratory function, but this must be of an accessory nature, for
the nymphs live after the removal of the processes, and indeed reproduce
them; the skin of these processes is harder than is usual in Insect gills.
In the nymph of _Euphaea_—a genus of Calepteryginae found in tropical
Asia—there are also external {423}abdominal gills, and in this case
respiration may, according to Hagen,[340] take place in four different
manners: (1) by ten pairs of stigmata; (2) by lateral branchiae well
furnished with tracheae; (3) by caudal branchiae; (4) by rectal branchiae.
It is further said that in this Insect the lateral branchiae persist in the
imago.
Although the means of respiration of the nymphs have been fairly well
ascertained, yet the mode in which the nymph is prepared for the sudden
change from the aquatic to aerial life is still obscure, the condition of
the stigmata not being thoroughly elucidated. It appears probable, however,
that the young nymph has no stigmata; that these organs appear in the
course of its development, being at first quite impervious, but becoming—at
any rate in the case of the larger and more important pair—open previous to
the final ecdysis. We have mentioned the contradictory opinions of Réaumur
and Dufour, and will now add the views of some modern investigators.
Oustalet says[341] that there are two pairs of spiracles in the nymphs; the
first pair is quite visible to the naked eye, and is situate between pro-
and meso-notum; it is in the nymph closed by a membrane. The other pair of
spiracles is placed above the posterior pair of legs, is small and
completely closed. He does not state what stage of growth was attained by
the nymphs he examined. Palmén was of opinion that not only thoracic but
abdominal spiracles exist in the nymph,[342] and that they are completely
closed so that no air enters them; he says that the spiracles have tracheae
connected with them, that at each moult the part closing the spiracles is
shed with some of the tracheal exuviae attached to it. The breathing
orifices are therefore for a short time at each ecdysis open, being
subsequently again closed by some exudation or secretion. This view of
Palmén's has been thought improbable by Hagen and Dewitz, who operated by
placing nymphs in alcohol or warm water and observing the escape of bubbles
from the spots where the supposed breathing orifices are situate. Both
these observers found much difference in the results obtained in the cases
of young and of old nymphs. Hagen concludes that the first pair of thoracic
spiracles are functionally active, and that abdominal stigmata exist though
{424}functionless; he appears to be of opinion that when the first thoracic
stigma is closed this is the result of the abutting against it of a closed
trachea. Dewitz found[343] that in the adult nymph of _Aeschna_ the
thoracic stigma is well developed, while the other stigmata—to what number
and in what position is not stated—are very small. In a half-grown Aeschnid
nymph he found the thoracic stigma to be present in an undeveloped form. On
placing a full-grown nymph in alcohol, gas escaped from the stigma in
question, but in immature nymphs no escape of gas occurred although they
were subjected to a severe test. A specimen that, when submitted to the
above-mentioned immersion, emitted gas, subsequently moulted, and
thereafter air escaped from the spiracle previously impervious. The
observations of Hagen and Dewitz are perhaps not so adverse to the views of
Palmén as has been supposed, so that it would not be a matter for surprise
if Palmén's views on this point should be shown to be quite correct.
The number of species of Odonata or Libellulidae that have been described
is somewhat less than two thousand, but constant additions are made to the
number, and when the smaller and more fragile forms from the tropics are
collected and worked out it will probably be found that the number of
existing species is somewhere between five and ten thousand. They are
distributed all over the world, but are most numerous in species in the
warmer regions, and their predominance in any one locality is very much
regulated by the existence of waters suitable for the early stages of their
lives.
A good work on the British Odonata is still a desideratum.[344] In Britain
about forty-six species are believed to be native. They are said to be of
late years less numerous than they used to be. Notwithstanding their great
powers of flight, dragon-flies are destroyed by birds of various kinds;
several hawks are said to be very fond of them, and _Merops persicus_ to
line its nest with their wings. The number of Insects killed by
dragon-flies in places where they are abundant must be enormous; the
nymphs, too, {425}are very destructive in the waters they inhabit, so that
dragon-flies have no doubt been no mean factor in maintaining that
important and delicate balance of life which it is so difficult for us to
appreciate. The nymphs are no doubt cannibals, and this may perhaps be an
advantage to the species, as the eggs are sometimes deposited in large
numbers in a limited body of water, where all must perish if the nymphs did
not, after exhausting other food, attack one another. Martin, speaking of
the Odonata of the Département de l'Indre in France, says:[345] "The eggs,
larvae, and nymphs are the prey of several fishes, snakes, newts,
Coleoptera, aquatic Hemiptera, and of some diving birds. Sometimes the
destruction is on a considerable scale, and one may notice the dragon-flies
of some piece of water to diminish gradually in numbers, while the animals
that prey on them increase, so that a species may for a time entirely
disappear in a particular spot, owing to the attacks of some enemy that has
been specially prosperous, and also eager in their pursuit. De Selys found
that from a pond filled with carp, roach, perch, and eels, several of the
dragon-fly denizens disappeared directly the bream was introduced." On the
other hand, there can be little doubt that the nymphs are sometimes
injurious to fish; it has been recorded that in a piscicultural
establishment in Hungary 50,000 young fishes were put into a pond in
spring; in the following autumn only fifty-four fish could be found, but
there were present an enormous quantity of dragon-fly nymphs.
Odonata are among the few kinds of Insects that are known to form swarms
and migrate. Swarms of this kind have been frequently observed in Europe
and in North America; they usually consist of species of the genus
_Libellula_, but species of various other genera also swarm, and sometimes
a swarm may consist of more than one species. _L. quadrimaculata_ is the
species that perhaps most frequently forms these swarms in Europe; a large
migration of this species is said to occur every year in the Charente
inférieure from north to south.[346] It is needless to say that the
instincts and stimuli connected with these migrations are not understood.
The nymphs are capable, under certain circumstances, of accommodating
themselves to very peculiar conditions of life. The Sandwich Islands are
extremely poor in stagnant waters, and {426}yet there exist in this remote
archipelago several highly peculiar species of Agrioninae. Mr. R. C. L.
Perkins has recently discovered that the nymphs of some of these are
capable of maintaining their existence and completing their development in
the small collections of water that accumulate in the leaves of some lilies
growing on dry land. These nymphs (Fig. 271) have a shorter mask than
occurs, we believe, in any other Odonata, and one would suppose that they
must frequently wait long for a meal, as they must be dependent on stray
Insects becoming immersed in these tiny reservoirs. The cannibal habits of
the Odonata probably stand these lily-dwellers in good stead; Mr. Perkins
found that there were sometimes two or three nymphs of different sizes
together, and we may suspect that it sometimes goes hard with the smaller
fry. The extension in the length of the body of one of these
lily-frequenting Agrions when it leaves the water for its aerial existence
is truly extraordinary.
[Illustration: FIG. 271.—Under side of Agrionid nymph, with short mask,
living in water in lilies. Hawaiian Islands. × 3.]
The Odonata have no close relations with any other group of Insects. They
were associated by Latreille with the Ephemeridae, in a family called
Subulicornia. The members of the two groups have, in fact, a certain
resemblance in some of the features of their lives, especially in the
sudden change, without intermediate condition, from aquatic to aerial life;
but in all important points of structure, and in their dispositions,
dragon-flies and may-flies are totally dissimilar, and there is no
intermediate group to connect them. We have already, said that the Odonata
consist of two very distinct divisions—Anisopterides and Zygopterides. The
former group comprises the subfamilies Gomphinae, Cordulegasterinae,
Aeschninae, Corduliinae, and Libellulinae,—Insects having the hinder wings
slightly larger than the anterior pair; while the Zygopterides consist of
only two subfamilies—Calepteryginae and Agrioninae; they have the wings of
the two pairs equal in size, or the hinder a little the smaller. The two
groups Gomphinae and Calepteryginae are each, in several respects, of lower
development than the others, and authorities are divided {427}in opinion as
to which of the two should be considered the more primitive. It is
therefore of much interest to find that there exists an Insect that shares
the characters of the two primitive subfamilies in a striking manner. This
Insect, _Palaeophlebia superstes_ (Fig. 272), has recently been discovered
in Japan, and is perhaps the most interesting dragon-fly yet obtained. De
Selys Longchamps refers it to the subfamily Calepteryginae, on account of
the nature of its wings; were the Insect, however, deprived of these
organs, no one would think of referring _Palaeophlebia_ to the group in
question, for it has the form, colour, and appearance of a Gomphine
Odonate. Moreover, the two sexes differ in an important character,—the form
of the head and eyes. In this respect the female resembles a Gomphine of
inferior development; while the male, by the shape and large size of the
ocular organs, may be considered to combine the characters of Gomphinae and
Calepteryginae. The Insect is very remarkable in colour, the large eyes
being red in the dead examples. We do not, however, know what may be their
colour during life, as only one pair of the species is known, and there is
no record as to the life-history and habits. De Selys considers the nearest
ally of this Insect to be _Heterophlebia dislocata_, a fossil dragon-fly
found in the Lower Lias of England.
[Illustration: FIG. 272.—_Palaeophlebia superstes._ A, The Insect with
wings of one side and with two legs removed; B, front view of head of
female; C, of male. (After De Selys.)]
Numerous fossil dragon-flies are known; the group is well represented in
the Tertiary strata, and specimens have been found in amber. In strata of
the Secondary age these Insects {428}have been found as far back as the
Lower Lias; their remains are said to exist in considerable variety in the
strata of that epoch, and some of them to testify to the existence at that
period of dragon-flies as highly specialised as those now living. According
to Hagen[347] _Platephemera antiqua_ and _Gerephemera simplex_, two
Devonian fossils, may be considered as dragon-flies; the evidence as to
this appears inadequate, and Brongniart refers the latter Insect to the
family Platypterides, and considers Platephemera to be more allied to the
may-flies.
One of the most remarkable of the numerous discoveries lately made in
fossil entomology is the finding of remains of huge Insects, evidently
allied to dragon-flies, in the Carboniferous strata at Commentry.
Brongniart calls these Insects Protodonates,[348] and looks on them as the
precursors of our Odonata. _Meganeura monyi_ was the largest of these
Insects, and measured over two feet across the expanded wings. If M.
Brongniart be correct in his restoration of this giant of the Insect world,
it much resembled our existing dragon-flies, but had a simple structure of
the thoracic segments, and a simpler system of wing-nervures. On p. 276 we
figured _Titanophasma fayoli_, considered by Scudder and Brongniart as
allied to the family Phasmidae, and we pointed out that this supposed
alliance must at best have been very remote. This view is now taken by M.
Brongniart himself,[349] he having removed the Insect from the
Protophasmides to locate it in the Protodonates near Meganeura. There
appears to be some doubt whether the wings supposed to belong to this
specimen were really such, or belonged rather to some other species.